Introduce new function real_path_if_valid()

The function is like real_path(), except that it returns NULL on error
instead of dying.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>

abspath.c

@@ -153,6 +153,11 @@ const char *real_path(const char *path)
 	return real_path_internal(path, 1);
 }
 
+const char *real_path_if_valid(const char *path)
+{
+	return real_path_internal(path, 0);
+}
+
 static const char *get_pwd_cwd(void)
 {
 	static char cwd[PATH_MAX + 1];