Fixed valgrind complain on usage of uninitialized data.

sql/field.cc

@@ -6202,16 +6202,15 @@ double Field_str::middle_point_pos(Field *min, Field *max)
   uchar minp_prefix[sizeof(ulonglong)];
   uchar maxp_prefix[sizeof(ulonglong)];
   ulonglong mp, minp, maxp;
-  uint dsz= min(field_length, sizeof(ulonglong) * charset()->mbmaxlen);
-  memset(mp_prefix, 0, sizeof(ulonglong));
-  memset(minp_prefix, 0, sizeof(ulonglong));
-  memset(maxp_prefix, 0, sizeof(ulonglong));
   my_strnxfrm(charset(), mp_prefix, sizeof(mp),
-              ptr + length_size(), dsz);
+              ptr + length_size(),
+              data_length());
   my_strnxfrm(charset(), minp_prefix, sizeof(minp),
-              min->ptr + length_size(), dsz);
+              min->ptr + length_size(),
+              min->data_length());
   my_strnxfrm(charset(), maxp_prefix, sizeof(maxp),
-              max->ptr + length_size(), dsz);
+              max->ptr + length_size(),
+              max->data_length());
   mp= char_prefix_to_ulonglong(mp_prefix);
   minp= char_prefix_to_ulonglong(minp_prefix);
   maxp= char_prefix_to_ulonglong(maxp_prefix);