target/semihosting_common: fix minor memory leak

Reported by clang static analyzer.

Change-Id: Ie663f49d92588c0d8b502cfdd8fc34004b308066
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5380
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>

src/target/semihosting_common.c

@@ -944,6 +944,8 @@ int semihosting_common(struct target *target)
 					uint8_t *fn1 = malloc(len1+1);
 					uint8_t *fn2 = malloc(len2+1);
 					if (!fn1 || !fn2) {
+						free(fn1);
+						free(fn2);
 						semihosting->result = -1;
 						semihosting->sys_errno = ENOMEM;
 					} else {