[QtScript/JavaScriptCore] Throw error when RegExp constructor is passed invalid flags

Patch: http://gitorious.org/+qtwebkit-developers/qtwebkit/qtwebkit-qtscript-integration/commit/dde214dfbe1f4cac023164318b6878d932478d80

ECMA-262, 5th edition, 15.10.4.1: "If F contains any character other than "g", "i", or "m", or if it contains the same character more than once, then throw a SyntaxError exception."

Patch needs testcase.

Attached alternative patch.

Suggested action: Create upstream bug, finish the patch. Drop the patch if it's rejected. (No major real-world JS could have depended on this behavior yet, otherwise it would have been fixed upstream already.)