tds: fix potential NULL dereference

The result of the realloc function can be NULL if the memory allocation fails.
Found by PostgresPro with Svace Static Analyzer
Signed-off-by: Maksim Korotkov <m.korotkov@postgrespro.ru>

Author: ProjectMutilation

src/tds/token.c

@@ -2541,7 +2541,13 @@ tds_alloc_get_string(TDSSOCKET * tds, char **string, size_t len)
 		*string = NULL;
 		return -1;
 	}
-	s = (char*) realloc(s, out_len + 1);
+	char *tmp = (char*) realloc(s, out_len + 1);
+	if (!tmp) {
+		free(s);
+		*string = NULL;
+		return -1;
+	}
+	s = tmp;
 	s[out_len] = '\0';
 	*string = s;
 	return 0;