Fix dangling freed pointer in pkcs7_free_signer_info

This may have been a use-after-free, but I haven't worked out whether it was
a problem or not. Even if it turns out to have been ok, keeping invalid
pointers around is fragile.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/pkcs7.c

@@ -353,6 +353,7 @@ static void pkcs7_free_signer_info( mbedtls_pkcs7_signer_info *signer )
         name_cur = name_cur->next;
         mbedtls_free( name_prv );
     }
+    signer->issuer.next = NULL;
 }
 
 /**