Added checks for uninitalized memory when writing to IO_CACHE

This was done to be able to track some cases of unallocated memory
in replication tests reported by MSAN.

include/my_sys.h

@@ -24,9 +24,7 @@ C_MODE_START
 
 
 #include <my_valgrind.h>
-
 #include <my_pthread.h>
-
 #include <m_ctype.h>                    /* for CHARSET_INFO */
 #include <stdarg.h>
 #include <typelib.h>
@@ -509,6 +507,7 @@ static inline int my_b_read(IO_CACHE *info, uchar *Buffer, size_t Count)
 
 static inline int my_b_write(IO_CACHE *info, const uchar *Buffer, size_t Count)
 {
+  MEM_CHECK_DEFINED(Buffer, Count);
   if (info->write_pos + Count <= info->write_end)
   {
     memcpy(info->write_pos, Buffer, Count);
@@ -530,6 +529,7 @@ static inline int my_b_get(IO_CACHE *info)
 
 static inline my_bool my_b_write_byte(IO_CACHE *info, uchar chr)
 {
+  MEM_CHECK_DEFINED(&chr, 1);
   if (info->write_pos >= info->write_end)
     if (my_b_flush_io_cache(info, 1))
       return 1;

mysys/mf_iocache.c

@@ -1566,6 +1566,8 @@ int my_b_append(IO_CACHE *info, const uchar *Buffer, size_t Count)
 {
   size_t rest_length,length;
 
+  MEM_CHECK_DEFINED(Buffer, Count);
+
   /*
     Assert that we cannot come here with a shared cache. If we do one
     day, we might need to add a call to copy_to_read_buffer().