patch_parse: reject empty path names

When parsing patch headers, we currently accept empty path names just
fine, e.g. a line "--- \n" would be parsed as the empty filename. This
is not a valid patch format and may cause `NULL` pointer accesses at a
later place as `git_buf_detach` will return `NULL` in that case.

Reject such patches as malformed with a nice error message.

src/patch_parse.c

@@ -69,6 +69,10 @@ static int parse_header_path_buf(git_buf *path, git_patch_parse_ctx *ctx, size_t
 {
 	int error;
 
+	if (!path_len)
+		return git_parse_err("patch contains empty path at line %"PRIuZ,
+				     ctx->parse_ctx.line_num);
+
 	if ((error = git_buf_put(path, ctx->parse_ctx.line, path_len)) < 0)
 		goto done;
 

tests/patch/parse.c

@@ -149,6 +149,13 @@ void test_patch_parse__lifetime_of_patch_does_not_depend_on_buffer(void)
 	git_patch_free(patch);
 }
 
+void test_patch_parse__binary_file_with_missing_paths(void)
+{
+	git_patch *patch;
+	cl_git_fail(git_patch_from_buffer(&patch, PATCH_BINARY_FILE_WITH_MISSING_PATHS,
+					  strlen(PATCH_BINARY_FILE_WITH_MISSING_PATHS), NULL));
+}
+
 void test_patch_parse__memory_leak_on_multiple_paths(void)
 {
 	git_patch *patch;

tests/patch/patch_common.h

@@ -906,6 +906,12 @@
 	"+bb\n" \
 	" c\n"
 
+#define PATCH_BINARY_FILE_WITH_MISSING_PATHS \
+	"diff --git  \n" \
+	"--- \n" \
+	"+++ \n" \
+	"Binary files "
+
 #define PATCH_MULTIPLE_OLD_PATHS \
 	"diff --git  \n" \
 	"---  \n" \