[sanitizer] Fix a crash in FP unwinder on ARM.

This change fixes 2 issues in the fast unwinder from r217079:
* A crash if a frame pointer points below current stack head, but
  inside the current thread stack limits. That memory may be
  unmapped. A check for this was lost in r217079.
* The last valid stack frame (the first one with an invalid next
  frame pointer) is always interpreted as a GCC layout frame. This
  results in garbled last PC in the (expected) case when the last
  frame has LLVM layout.