Fix VC++ warning, "conversion from ha_checksum to char"

extra/comp_err.c

@@ -639,7 +639,7 @@ static struct message *find_message(struct errors *err, const char *lang,
 
 static char checksum_format_specifier(const char* msg)
 {
-  char chksum= 0;
+  ha_checksum chksum= 0;
   const char* p= msg;
   const char* start= 0;
   int num_format_specifiers= 0;