Fix RegExp.prototype.compile error case

test/mjsunit/regexp-compile.js

 // Copyright 2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 24 matching lines @@
 
 re.compile("(x)");
 
 assertEquals(["x", "x"], re.exec("axyb"));
 
 re.compile("(y)");
 
 assertEquals(["y", "y"], re.exec("axyb"));
 
 assertEquals(2, re.compile.length);
+
+// If RegExp parsing fails, the RegExp is not modified
+var r = /./; try { r.compile('? invalid'); } catch(err){}
+assertEquals("/./", r.toString());