Check dynamic slot function for nullness before evaluation

Check dynamic slot function for nullness before evaluation

Previously, we didn't check whether the function object handle
associated with a dynamic slot's method index was null before
attempting to evaluate it, which could cause a crash in some
circumstances.  This change also adds better error reporting
during function compilation.

Task-number: QTBUG-24064
Task-number: QTBUG-24037
Task-number: QTBUG-23387
Change-Id: I3c5e35e8c16187870125736013a5935fcc5cb1f2
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>