The changes in d6f96eda enabled handling cases
where the stack may be non-contiguous or not fully symbolicated (e.g. in cases
of alternate signal stacks).  However, it did not properly honour the requests
from the caller to perform a strict unwinding where such frames are terminated
upon the discontinuity.  This repairs that condition.

Hoist the alignment check since that is safe to perform early.

PiperOrigin-RevId: 464160529
Change-Id: Ic65645928ec60c2a3b4844f3abd4fed1b991edab