reachableroots: return NULL if we're throwing an exception
Based on my reading of [0] and surrounding sections, if we want an
exception to be properly raised when something goes wrong in the C
code, we need to make sure we return NULL here. Do so.
https://docs.python.org/2/extending/extending.html#back-to-the-example
$ hg init
$ echo a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository 'subrepo'
[255]