Missing canary check

[Liberalsin]

The check to see if the stack was smashed also needs to be at the end of coru_resume(). It's not enough to put it only in coru_yield().

// check canary, if this fails a stack overflow occured
CORU_ASSERT(!coru_active->canary ||
        *coru_active->canary == (uintptr_t)0x636f7275);