Avoid use of unitialized variable.  Fixes addhunk2 test with vm.malloc_conf=J.

Index: src/rediff.c
--- src/rediff.c.orig
+++ src/rediff.c
@@ -842,6 +842,7 @@ static int rediff (const char *original, const char *e
 		newhunk->filepos = pos;
 		newhunk->line_in_diff = linenum;
 		newhunk->num_lines = 0;
+		newhunk->discard_offset = 0;
 
 		if (!strncmp (line, "--- ", 4)) {
 			struct file_info *info = xmalloc (sizeof *info);