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.