Fix conversion warnings/errors.

sql/field.h

@@ -3319,7 +3319,7 @@ class Field_blob :public Field_longstr {
   {
     store_length(ptr, packlength, number);
   }
-  inline uint32 get_length(uint row_offset= 0) const
+  inline uint32 get_length(my_ptrdiff_t row_offset= 0) const
   { return get_length(ptr+row_offset, this->packlength); }
   uint32 get_length(const uchar *ptr, uint packlength) const;
   uint32 get_length(const uchar *ptr_arg) const

unittest/sql/mf_iocache-t.cc

@@ -208,7 +208,7 @@ void mdev10259()
   res= my_b_flush_io_cache(&info, 1);
   ok(res == 0, "flush" INFO_TAIL);
 
-  ulong saved_pos=  my_b_tell(&info);
+  my_off_t saved_pos=  my_b_tell(&info);
   res= reinit_io_cache(&info, READ_CACHE, 0, 0, 0);
   ok(res == 0, "reinit READ_CACHE" INFO_TAIL);