mpatch: avoid integer overflow in combine() (SEC)
All the callers of this function can handle a NULL return, so that
appears to be the "safe" way to report an error.
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate