make prefix_path() never return NULL

There are 9 places where prefix_path is called, and only in one of
them the returned pointer was checked to be non-zero and only to
call exit(128) as it is usually done by die(). In other 8 places,
the returned value was not checked and it caused SIGSEGV when a
path outside of the working tree was used. For instance, running
  git update-index --add /some/path/outside
caused SIGSEGV.

This patch changes prefix_path() to die if the path is outside of
the repository, so it never returns NULL.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

setup.c

@@ -110,9 +110,7 @@ const char *prefix_path(const char *prefix, int len, const char *path)
 		if (strncmp(sanitized, work_tree, len) ||
 		    (sanitized[len] != '\0' && sanitized[len] != '/')) {
 		error_out:
-			error("'%s' is outside repository", orig);
-			free(sanitized);
-			return NULL;
+			die("'%s' is outside repository", orig);
 		}
 		if (sanitized[len] == '/')
 			len++;
@@ -216,10 +214,7 @@ const char **get_pathspec(const char *prefix, const char **pathspec)
 	prefixlen = prefix ? strlen(prefix) : 0;
 	while (*src) {
 		const char *p = prefix_path(prefix, prefixlen, *src);
-		if (p)
-			*(dst++) = p;
-		else
-			exit(128); /* error message already given */
+		*(dst++) = p;
 		src++;
 	}
 	*dst = NULL;