Use palloc() instead of palloc0() in buildGpQueryString() (#10875)

The buildGpQueryString() function should use palloc() instead of palloc0()
to allocate the buffer for holding the serialized query string, thus
avoiding the unnecessary cost of setting all bytes in buffer to zero.
Reviewed-by: NHeikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: NAshwin Agrawal <aashwin@vmware.com>

src/backend/cdb/dispatcher/cdbdisp_query.c

@@ -910,7 +910,7 @@ buildGpQueryString(DispatchCommandQueryParms *pQueryParms,
 		sizeof(resgroupInfo.len) +
 		resgroupInfo.len;
 
-	shared_query = palloc0(total_query_len);
+	shared_query = palloc(total_query_len);
 
 	pos = shared_query;