ASN.1: Fix check for strdup() success

It seems there is a misprint in the check of strdup() return code that
can lead to NULL pointer dereference.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 4520c6a49a ("X.509: Add simple ASN.1 grammar compiler")
Signed-off-by: Ekaterina Orlova <vorobushek.ok@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: keyrings@vger.kernel.org
Cc: linux-kbuild@vger.kernel.org
Link: https://lore.kernel.org/r/20230315172130.140-1-vorobushek.ok@gmail.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/asn1_compiler.c

@@ -625,7 +625,7 @@ int main(int argc, char **argv)
 	p = strrchr(argv[1], '/');
 	p = p ? p + 1 : argv[1];
 	grammar_name = strdup(p);
-	if (!p) {
+	if (!grammar_name) {
 		perror(NULL);
 		exit(1);
 	}