[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.