core: Fix a strtoul to strtol
core: Fix a strtoul to strtol

* src/assuan-support.c (my_spawn): Fix using strtoul for
a long.

--
This was commented on in129def87b262 and is correct.
It is signed here to better handle cases where an
invalid handle value (-1) would be passed.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme