Decimal field fix overflow

sql/field.cc:
  Fixes for Decimal crash bug patch according to Monty's comments ?

sql/field.cc

@@ -41,6 +41,11 @@
 #include <floatingpoint.h>
 #endif
 
+// Maximum allowed exponent value for converting string to decimal
+#define MAX_EXPONENT 1024
+
+
+
 /*****************************************************************************
   Instansiate templates and static variables
 *****************************************************************************/
@@ -367,7 +372,6 @@ void Field_decimal::store(const char *from,uint len)
   /* The pointer where the field value starts (i.e., "where to write") */
   char *to=ptr;
   uint tmp_dec, tmp_uint;
-  ulonglong tmp_ulonglong;
   /*
     The sign of the number : will be 0 (means positive but sign not
     specified), '+' or '-'
@@ -382,7 +386,6 @@ void Field_decimal::store(const char *from,uint len)
  /* The sign of the exponent : will be 0 (means no exponent), '+' or '-' */
   char expo_sign_char=0;
   uint exponent=0;                                // value of the exponent
-  ulonglong exponent_ulonglong=0;
   /*
     Pointers used when digits move from the left of the '.' to the
     right of the '.' (explained below)
@@ -485,14 +488,13 @@ void Field_decimal::store(const char *from,uint len)
     */
     for (;from!=end && isdigit(*from); from++)
     {
-      exponent_ulonglong=10*exponent_ulonglong+(ulonglong)(*from-'0');
-      if (exponent_ulonglong>(ulonglong)UINT_MAX)
+      exponent=10*exponent+(*from-'0');
+      if (exponent>MAX_EXPONENT)
       {
-        exponent_ulonglong=(ulonglong)UINT_MAX;
+        exponent=MAX_EXPONENT;
         break;
       }
     }
-    exponent=(uint)(exponent_ulonglong);
   }
   
   /*
@@ -534,21 +536,21 @@ void Field_decimal::store(const char *from,uint len)
   */
   
   /* 
-     Below tmp_ulongulong cannot overflow,
+     Below tmp_uint cannot overflow with small enough MAX_EXPONENT setting,
      as int_digits_added_zeros<=exponent<4G and 
      (ulonglong)(int_digits_end-int_digits_from)<=max_allowed_packet<=2G and
      (ulonglong)(frac_digits_from-int_digits_tail_from)<=max_allowed_packet<=2G
   */
 
   if (!expo_sign_char)
-    tmp_ulonglong=(ulonglong)tmp_dec+(ulonglong)(int_digits_end-int_digits_from);
+    tmp_uint=tmp_dec+(uint)(int_digits_end-int_digits_from);
   else if (expo_sign_char == '-') 
   {
     tmp_uint=min(exponent,(uint)(int_digits_end-int_digits_from));
     frac_digits_added_zeros=exponent-tmp_uint;
     int_digits_end -= tmp_uint;
     frac_digits_head_end=int_digits_end+tmp_uint;
-    tmp_ulonglong=(ulonglong)tmp_dec+(ulonglong)(int_digits_end-int_digits_from);     
+    tmp_uint=tmp_dec+(uint)(int_digits_end-int_digits_from);     
   }
   else // (expo_sign_char=='+') 
   {
@@ -575,9 +577,9 @@ void Field_decimal::store(const char *from,uint len)
 	int_digits_added_zeros=0;
       }
     }
-    tmp_ulonglong=(ulonglong)tmp_dec+(ulonglong)(int_digits_end-int_digits_from)
-                 +(ulonglong)(frac_digits_from-int_digits_tail_from)+
-                 (ulonglong)int_digits_added_zeros;
+    tmp_uint=tmp_dec+(int_digits_end-int_digits_from)
+                 +(uint)(frac_digits_from-int_digits_tail_from)+
+                 int_digits_added_zeros;
   }
   
   /*
@@ -588,7 +590,7 @@ void Field_decimal::store(const char *from,uint len)
     If the sign is defined and '-', we need one position for it
   */
 
-  if ((ulonglong)field_length < tmp_ulonglong + (ulonglong) (sign_char == '-')) 
+  if (field_length < tmp_uint + (sign_char == '-')) 
   //the rightmost sum above cannot overflow
   {
     // too big number, change to max or min number
@@ -596,11 +598,6 @@ void Field_decimal::store(const char *from,uint len)
     return;
   }
  
-  /*
-    If the above test was ok, then tmp_ulonglong<4G and the following cast is valid
-  */
-  tmp_uint=(uint)tmp_ulonglong; 
-
   /*
     Tmp_left_pos is the position where the leftmost digit of
     the int_% parts will be written