btrfs-progs: string-table: fix memory leak on exit path in table_vprintf()

Reported by 'gcc -fanalyzer:
common/string-table.c:62:17: warning: leak of ‘msg’ [CWE-401] [-Wanalyzer-malloc-leak]

The 'msg' still allocated when returning from the function due to error,
free it.

Signed-off-by: David Sterba <dsterba@suse.com>

common/string-table.c

@@ -57,6 +57,7 @@ char *table_vprintf(struct string_table *tab, unsigned int column, unsigned int
 		return NULL;
 
 	if (column >= tab->ncols || row >= tab->nrows) {
+		free(msg);
 		error("attempt to write outside of table: col %u row %u fmt %s",
 			column, row, fmt);
 		return NULL;