Fix mkDirRecursively

Change-Id: Ie09fd666ca208994f22e9f83af6072f4d5e6aa1a

Author: xuwei06

paddle/utils/Util.cpp

@@ -289,6 +289,7 @@ void mkDir(const char* filename) {
 void mkDirRecursively(const char* dir) {
   struct stat sb;
 
+  if (*dir == 0) return;  // empty string
   if (!stat(dir, &sb)) return;
 
   mkDirRecursively(path::dirname(dir).c_str());