avoid asking ?alloc() for zero bytes.

Avoid asking for zero bytes when that change simplifies overall
logic.  Later we would change the wrapper to ask for 1 byte on
platforms that return NULL for zero byte request.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.c

@@ -504,9 +504,9 @@ static void prepare_temp_file(const char *name,
 		}
 		if (S_ISLNK(st.st_mode)) {
 			int ret;
-			char *buf, buf_[1024];
-			buf = ((sizeof(buf_) < st.st_size) ?
-			       xmalloc(st.st_size) : buf_);
+			char buf[PATH_MAX + 1]; /* ought to be SYMLINK_MAX */
+			if (sizeof(buf) <= st.st_size)
+				die("symlink too long: %s", name);
 			ret = readlink(name, buf, st.st_size);
 			if (ret < 0)
 				die("readlink(%s)", name);