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: 4520c6a49af8 ("X.509: Add simple ASN.1 grammar compiler")
Signed-off-by: Ekaterina Orlova <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: James Bottomley <[e-mail]>
Cc: Jarkko Sakkinen <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

scripts/asn1_compiler.c

···
 	p = strrchr(argv[1], '/');
 	p = p ? p + 1 : argv[1];
 	grammar_name = strdup(p);
-	if (!p) {
+	if (!grammar_name) {
 		perror(NULL);
 		exit(1);
 	}