Runtime: Fixed dereferance and/or free of uninitialized pointer.
The PIKE_FRAME_SAVE_LOCALS flag was unintentionally propagated
to the current frame by unlink_previous_frame(), and the
corresponding pointer is not cleared by alloc_pike_frame(), so
previously freed memory could be written to by eg F_SAVE_LOCALS,
and would be freed again by eg really_free_pike_frame().
Fixes testsuite failures.
Also adds some related paranoioa regarding the flag.