build: fix memory leaks

Fix the memory leaks found by clang-3.1

Change-Id: Iaae68627ef599c324c9c9ee5737c22e92512862d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/775
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/flash/mflash.c

@@ -770,6 +770,7 @@ COMMAND_HANDLER(mg_write_cmd)
 	int retval = fileio_size(&fileio, &filesize);
 	if (retval != ERROR_OK) {
 		fileio_close(&fileio);
+		free(buffer);
 		return retval;
 	}