Fix bug : sometimes failed to get root CA

Fix bug : sometimes failed to get root CA

[Problem]      Sometimes failed to complete cert chain
[Cause]        getNameHash function return type is std::string.
               But searchByHash function get returned value and use it as .c_str() directly.
               So rvalue isn't saved properly and sometimes it goes to be empty.
[Solution]     Get returned std::string to proper variable(lvalue with std::string)
[Verification] Try 50 times to install sample tpk and no error occured.

Change-Id: Ic4b6e72c0f791546f04f7b37c6f2d1c9c40d1f8a
Signed-off-by: Kyungwook Tak <k.tak@samsung.com>