If a program has had its text section split into multiple
sections then the ftab that is built is based on addresses
prior to splitting.  That means all the function addresses
are there and correct because of relocation but the
but the computed idx won't always match up quite right and
in some cases go beyond the end of the table, causing a panic.

To resolve this, determine if the idx is too large and if it is,
set it to the last index in ftab.  Then search backward to find the
matching function address.

Fixes #17854

Change-Id: I6940e76a5238727b0a9ac23dc80000996db2579a
Reviewed-on: https://go-review.googlesource.com/32972Reviewed-by: David Chase <drchase@google.com>