ARM: backtrace: avoid crash on large invalid fp value

Wraparound on invalid frame pointer (signed) -4 <= fp <= -1
defeats check for too high.

Change-Id: Iaccb5d380e989660c06c0379a5e4ee31d9f84966
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Aleksandar Momiroski <alex.momir@ti.com>