mpatch: introduce a safeadd() helper to work around UB int overflow
We're about to make extensive use of this. This change duplicates some
stdbool.h portability hacks from cext/util.h. We should probably clean
that up in the future, but we'll skip that for now in order to make
security backports easier.
mpatch: ensure fragment start isn't past the end of orig (SEC)
Caught by oss-fuzz fuzzer during development.
This defect is OVE-
20180430-0004. A CVE has not been obtained as of
this writing.