[Reg] Crash / incorrect result when setting custom scope chain for a native context

Against 4.6, the attached program crashes.
Against 4.7, it gives the output

"" (expected: 123)
"123" (expected: empty string)

Which is not the same as against 4.5.
An assignment in the custom scope that causes a new variable to be created (i.e. not prefixed by "var") should create the variable on the global object, not on the inner scope.

Commenting out the "#define BUG2" line exposes another bug, which is that if the activation object of the nested context is popped using QScriptContext::popScope(), but no new scope object is pushed, subsequently QScriptContext::popContext() will crash.