patch_parse: fix parsing addition/deletion of file with space

[pks-t]

The diff header format is a strange beast in that it is inherently
unparseable in an unambiguous way. While parsing

a/file.txt b/file.txt

is obvious and trivially doable, parsing a diff header of

a/file b/file ab.txt b/file b/file ab.txt

is not (but in fact valid and created by git.git).

Due to that, we have relaxed our diff header parser in commit 80226b5
(patch_parse: allow parsing ambiguous patch headers, 2017-09-22), so
that we started to bail out when seeing diff headers with spaces in
their file names. Instead, we try to use the "---" and "+++" lines,
which are unambiguous.

In some cases, though, we neither have a useable file name from the
header nor from the "---" or "+++" lines. This is the case when we have
a deletion or addition of a file with spaces: the header is unparseable
and the other lines will simply show "/dev/null". This trips our parsing
logic when we try to extract the prefix (the "a/" part) that is being
used in the path line, where we unconditionally try to dereference a
NULL pointer in such a scenario.

We can fix this by simply not trying to parse the prefix in cases where
we have no useable path name. That'd leave the parsed patch without
either old_prefix or new_prefix populated. But in fact such cases
are already handled by users of the patch object, which simply opt to
use the default prefixes in that case.

[ethomson]

Thanks @eaigner for reporting the issue and @pks-t for the fix.