patch_parse: fix parsing addition/deletion of file with space

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 80226b5f
(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.

src/patch_parse.c

@@ -928,8 +928,8 @@ static int check_filenames(git_patch_parsed *patch)
 	prefixed_old = (!added && patch->old_path) ? patch->old_path : patch->header_old_path;
 	prefixed_new = (!deleted && patch->new_path) ? patch->new_path : patch->header_new_path;
 
-	if (check_prefix(&patch->old_prefix, &old_prefixlen, patch, prefixed_old) < 0 ||
-	    check_prefix(&patch->new_prefix, &new_prefixlen, patch, prefixed_new) < 0)
+	if ((prefixed_old && check_prefix(&patch->old_prefix, &old_prefixlen, patch, prefixed_old) < 0) ||
+	    (prefixed_new && check_prefix(&patch->new_prefix, &new_prefixlen, patch, prefixed_new) < 0))
 		return -1;
 
 	/* Prefer the rename filenames as they are unambiguous and unprefixed */