Catch SIGBUS in HeapWalker

SIGBUS shouldn't happen, since all of the regions being walked
were previously read out of /prod/pid/maps, but it seems to happen
rarely.  Catch it so it can produce a better log message.

Bug: 128983715
Test: atest memunreachable_test
Change-Id: I82d3941a382a0451c0dda12f5eb849ad8d697bc9