core: fix ASAN_START_SWITCH_FIBER() usage

The `__sanitizer_start_switch_fiber()` function takes a pointer as the
first argument to store the current fake stack if there is one (it is
necessary when stack-use-after-return detection is enabled). When leaving a
fiber definitely, NULL must be passed so that the fake stack is destroyed.

Before this patch, NULL was passed for dead fibers, however this is wrong
for dead fibers that are recycled and resumed. In such cases ASAN destroys
the fake stack, and the fiber crashes trying to use it in `fiber_yield()`
upon return from `coro_transfer()`.

Closes tarantool/tarantool-qa#321

NO_DOC=bugfix
NO_TEST=tested by test-release-asan workflow