Ensure handle is valid and pm_errno is reset when calling into API

Ensure handle is valid and pm_errno is reset when calling into API

We didn't do due diligence before and ensure prior pm_errno values
weren't influencing what happened in further ALPM calls. I observed one
case of early setup code setting pm_errno to PM_ERR_WRONG_ARGS and that
flag persisting the entire time we were calling library code.

Add a new CHECK_HANDLE() macro that does two things: 1) ensures the
handle variable passed to it is non-NULL and 2) clears any existing
pm_errno flag set on the handle. This macro can replace many places we
used the ASSERT(handle != NULL, ...) pattern before.

Several other other places only need a simple 'set to zero' of the
pm_errno field.

Signed-off-by: Dan McGee <dan@archlinux.org>