Fixed potential memory corruption in njs_function_frame_invoke().

Fixed potential memory corruption in njs_function_frame_invoke().

It is a bug to cast vm->top_frame which is (njs_native_frame_t *) type,
to (njs_frame_t *) in general context.  The cast is allowed only for the
lambda frames.

The bug did not manifest itself previously because native frames were
allocated with NJS_NATIVE_FRAME_SIZE, which is sizeof(njs_value_t) aligned
size of njs_native_frame_t (on 64bit platform are 80 and 72 bytes
respectively). njs_frame_t contains njs_native_frame_t as a first field.
The frame->retval assignment for native frames resulted in 8 padding bytes
were used as a storage space for the retval field.

The bug becomes visible when the size of njs_native_frame_t changes.

The issue was introduced in 540f03725df2.