hash_object: correction for zero length file

The check whether size is zero was done after if size <= SMALL_FILE_SIZE,
as result, zero size case was never triggered. Instead zero length file
was treated as any other small file. This did not caused any problem, but
if we have a special case for size equal to zero, it is better to make it
work and avoid redundant malloc().

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2448,6 +2448,8 @@ int index_fd(unsigned char *sha1, int fd, struct stat *st, int write_object,
 		else
 			ret = -1;
 		strbuf_release(&sbuf);
+	} else if (!size) {
+		ret = index_mem(sha1, NULL, size, write_object, type, path);
 	} else if (size <= SMALL_FILE_SIZE) {
 		char *buf = xmalloc(size);
 		if (size == read_in_full(fd, buf, size))
@@ -2456,12 +2458,11 @@ int index_fd(unsigned char *sha1, int fd, struct stat *st, int write_object,
 		else
 			ret = error("short read %s", strerror(errno));
 		free(buf);
-	} else if (size) {
+	} else {
 		void *buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
 		ret = index_mem(sha1, buf, size, write_object, type, path);
 		munmap(buf, size);
-	} else
-		ret = index_mem(sha1, NULL, size, write_object, type, path);
+	}
 	close(fd);
 	return ret;
 }