[Runtime] Fix using uninitialized variable "is_valid_" for WGT package
extracting.

The uninitialized variable "is_valid_" in package.cc(h) may lead to
error when installing a WGT package, and it should be initialized before
using it in Extract method.
Merge pull request #1756 from takethathe/fix_package_extract

[Runtime] Fix using uninitialized variable "is_valid_" for WGT package extracting.