V4: fix ArrayPrototype::method_isArray() build break

With a recent Clang in C++11 mode:
error: incompatible operand types ('QV4::ArrayObject *' and 'bool')

Task-number: QTBUG-33706
Change-Id: I7bd4fe01176745fb6f8dbdf8f271edb7121eb35e
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Fix compilation on Android with 4.8 toolchain

Change-Id: I1ce4ec8c9c671f0130c1530c772c1dd74f1fb1f4
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>