rb_io_each_codepoint: 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.

io.c

@@ -4173,8 +4173,7 @@ rb_io_each_codepoint(VALUE io)
 	    rb_yield(UINT2NUM(c));
 	}
 	else if (MBCLEN_INVALID_P(r)) {
-	  invalid:
-	    rb_raise(rb_eArgError, "invalid byte sequence in %s", rb_enc_name(enc));
+            goto invalid;
 	}
 	else if (MBCLEN_NEEDMORE_P(r)) {
 	    char cbuf[8], *p = cbuf;
@@ -4197,6 +4196,9 @@ rb_io_each_codepoint(VALUE io)
 	}
     }
     return io;
+
+  invalid:
+    rb_raise(rb_eArgError, "invalid byte sequence in %s", rb_enc_name(enc));
 }
 
 /*