signm2signo: do not goto into a branch

I'm not necessarily against every goto in general, but jumping into a
branch is definitely a bad idea.  Better refactor.

signal.c

@@ -243,6 +243,20 @@ signm2signo(VALUE *sig_ptr, int negative, int exit, int *prefix_ptr)
 	    prefix += signame_prefix_len;
     }
     if (len <= (long)prefix) {
+        goto unsupported;
+    }
+
+    if (prefix_ptr) *prefix_ptr = prefix;
+    nmlen = len - prefix;
+    nm += prefix;
+    if (nmlen > LONGEST_SIGNAME) goto unsupported;
+    FOREACH_SIGNAL(sigs, !exit) {
+	if (memcmp(sigs->signm, nm, nmlen) == 0 &&
+	    sigs->signm[nmlen] == '\0') {
+	    return negative ? -sigs->signo : sigs->signo;
+	}
+    }
+
   unsupported:
     if (prefix == signame_prefix_len) {
         prefix = 0;
@@ -260,19 +274,6 @@ signm2signo(VALUE *sig_ptr, int negative, int exit, int *prefix_ptr)
     }
     rb_raise(rb_eArgError, "unsupported signal `%.*s%"PRIsVALUE"'",
              prefix, signame_prefix, vsig);
-    }
-
-    if (prefix_ptr) *prefix_ptr = prefix;
-    nmlen = len - prefix;
-    nm += prefix;
-    if (nmlen > LONGEST_SIGNAME) goto unsupported;
-    FOREACH_SIGNAL(sigs, !exit) {
-	if (memcmp(sigs->signm, nm, nmlen) == 0 &&
-	    sigs->signm[nmlen] == '\0') {
-	    return negative ? -sigs->signo : sigs->signo;
-	}
-    }
-    goto unsupported;
 }
 
 static const char*