libopendkim: set sig_signalg before feature checks in dkim_siglist_setup()

When opendkim is compiled without Ed25519 support and receives a message
signed with ed25519-sha256, sig_signalg was left at its memset default
(0 = DKIM_SIGN_RSASHA1) because the continue statement in the feature
check jumped past the assignment.  The AR header would then incorrectly
report header.a=rsa-sha1 instead of header.a=ed25519-sha256.

Assign sig_signalg immediately when signalg is decoded from the a= tag,
before any continue that might skip the signature.

Fixes #27

Author: thegushi

libopendkim/dkim.c

@@ -2121,8 +2121,9 @@ dkim_siglist_setup(DKIM *dkim)
 		}
 		else
 		{
-			signalg = dkim_name_to_code(dkim_table_algorithms,
-			                            (char *) param);
+			dkim->dkim_siglist[c]->sig_signalg = signalg =
+				dkim_name_to_code(dkim_table_algorithms,
+				                  (char *) param);
 
 			if (signalg == -1)
 			{
@@ -2165,7 +2166,6 @@ dkim_siglist_setup(DKIM *dkim)
 				/* NOTREACHED */
 			}
 
-			dkim->dkim_siglist[c]->sig_signalg = signalg;
 			dkim->dkim_siglist[c]->sig_hashtype = hashtype;
 		}