Fix SIGSEGV on unmerged files in git-diff-files -p

NULL is not considered a VALID pathspec.

diffcore.h

@@ -33,7 +33,7 @@ struct diff_filespec {
 				  * if false, use the name and read from
 				  * the filesystem.
 				  */
-#define DIFF_FILE_VALID(spec) (((spec)->mode) != 0)
+#define DIFF_FILE_VALID(spec) ((spec) && ((spec)->mode) != 0)
 	unsigned should_free : 1; /* data should be free()'ed */
 	unsigned should_munmap : 1; /* data should be munmap()'ed */
 };