transcode.c: fix segv in String.encode!

* transcode.c (str_transcode0): fix segv in String.encode!.  now
  rb_str_scrub() can return nil.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43504 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

transcode.c

@@ -2699,7 +2699,9 @@ str_transcode0(int argc, VALUE *argv, VALUE *self, int ecflags, VALUE ecopts)
 		if (!NIL_P(ecopts)) {
 		    rep = rb_hash_aref(ecopts, sym_replace);
 		}
-		*self = rb_str_scrub(str, rep);
+		dest = rb_str_scrub(str, rep);
+		if (NIL_P(dest)) dest = str;
+		*self = dest;
 		return dencidx;
 	    }
             return NIL_P(arg2) ? -1 : dencidx;