fault_handler: use SafeCopy to decode x86 instruction length.

Prevent a crash in the x86 fault handler when pc points to garbage and
we try to figure out its instruction length.

Bug: http://b/30836730
Test: m test-art-host
Change-Id: Ic1fbb8856e30140f0e1ebc9caccf9559e88ff137
1 file changed