rerere: Fix use of an empty strbuf.buf

The code incorrectly assumed that strbuf.buf is always an
allocated piece of memory that has NUL at offset strbuf.len.
That assumption does not hold for a freshly initialized empty
strbuf.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-rerere.c

@@ -113,8 +113,10 @@ static int handle_file(const char *path,
 				fputs(">>>>>>>\n", out);
 			}
 			if (sha1) {
-				SHA1_Update(&ctx, one.buf, one.len + 1);
-				SHA1_Update(&ctx, two.buf, two.len + 1);
+				SHA1_Update(&ctx, one.buf ? one.buf : "",
+					    one.len + 1);
+				SHA1_Update(&ctx, two.buf ? two.buf : "",
+					    two.len + 1);
 			}
 			strbuf_reset(&one);
 			strbuf_reset(&two);