MySQL: don't allocate 1-byte buffers for BLOBs

We set the buffer length to 0 for blobs, as we need to do it for each
row, in bindBlobs() (apparently a workaround for MySQL 4.1.8 API). That
function was deleting the buffer and reallocating.

Change-Id: I4a40ccbd3321467a8429fffd169b06422612ca13
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/plugins/sqldrivers/mysql/qsql_mysql.cpp

@@ -364,7 +364,7 @@ bool QMYSQLResultPrivate::bindInValues()
         bind->length = &f.bufLength;
         bind->is_unsigned = fieldInfo->flags & UNSIGNED_FLAG ? 1 : 0;
 
-        char *field = new char[bind->buffer_length + 1]{};
+        char *field = bind->buffer_length ? new char[bind->buffer_length + 1]{} : nullptr;
         bind->buffer = f.outField = field;
 
         ++i;