pki: NULL check pki_signature_from_rsa_blob result

Check for a potential NULL result from `pki_signature_from_rsa_blob`
in `pki_signature_from_blob`.  Otherwise the following `sig->type_c`
will result in a segfault.

Introduced in 7f83a1efae6a7da19e18268d6298fc11b4e68c57.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/pki_crypto.c

@@ -1681,6 +1681,9 @@ ssh_signature pki_signature_from_blob(const ssh_key pubkey,
         case SSH_KEYTYPE_RSA:
         case SSH_KEYTYPE_RSA1:
             sig = pki_signature_from_rsa_blob(pubkey, sig_blob, sig);
+            if (sig == NULL) {
+                return NULL;
+            }
             sig->type_c = ssh_key_signature_to_char(type, hash_type);
             break;
         case SSH_KEYTYPE_ECDSA:

src/pki_mbedcrypto.c

@@ -917,6 +917,9 @@ ssh_signature pki_signature_from_blob(const ssh_key pubkey,
     switch(type) {
         case SSH_KEYTYPE_RSA:
             sig = pki_signature_from_rsa_blob(pubkey, sig_blob, sig);
+            if (sig == NULL) {
+                return NULL;
+            }
             sig->type_c = ssh_key_signature_to_char(type, hash_type);
             break;
         case SSH_KEYTYPE_ECDSA: {