ASN.1: fix open failure check on headername

The check for a failed open on headername is incorrectly checking
on the out FILE pointer rather than the hdr. Fix this.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Howells <dhowells@redhat.com>

scripts/asn1_compiler.c

@@ -650,7 +650,7 @@ int main(int argc, char **argv)
 	}
 
 	hdr = fopen(headername, "w");
-	if (!out) {
+	if (!hdr) {
 		perror(headername);
 		exit(1);
 	}