Reset user func trampoline values FFI may overwrite

[iluuu1994]

This was revealed by #10880 by changing padding and now overwriting last_var which broke stack size calculation.

The values are overwritten by FFI here:

php-src/ext/ffi/ffi.c

Lines 2168 to 2172 in c58c266

	func->internal_function.handler = ZEND_FN(ffi_trampoline);
	func->internal_function.module = NULL;
	func->internal_function.reserved[0] = type;
	func->internal_function.reserved[1] = *(void**)cdata->ptr;

A more future-proof solution might be to memset the entire function but I wanted to avoid adding that overhead without feedback first.

[dstogov]

Looks fine.
Actually, last_var must be set to zero at first place.

Previously we didn't set it because of calloc() or because it was never set by anything else in reused EG(trampoline), but when we added ext/FFI, it started to work out-of the box only because of "lucky" structure layout and we forgot to add that last_var initialization.

[iluuu1994]

Actually, last_var must be set to zero at first place.

Sorry, just so there's no misunderstanding: Do you mean it needs to be moved up further or are you reiterating why this needs to be added? Theoretically we could move this into the if (EXPECTED(EG(trampoline).common.function_name == NULL)) { branch.

[dstogov]

no, the C code is exactly right. The new comments may be confusing for people who are not familiar with the problem.

It's better to add comments that EG(trampoline) may be reused in different places (e.g. FFI) and all sensitive fields must be reinitialized.

[iluuu1994]

EG(trampoline) is reused from other places, like FFI (e.g. zend_ffi_cdata_get_closure()) where it is used as an internal function. It may overwrite fields that don't belong to common, thus overwriting zend_op_array specific data, most significantly last_var. We need to reset this value so that it doesn't contain garbage when the engine allocates space for the next stack frame. This didn't cause any issues until now due to "lucky" structure layout.

@dstogov Is that better?