Fix nullpointer dereference in libnativeloader

The `char* path` argument of OpenNativeLibrary() can be nullptr. We are
constructing std::string from the path, which is a bug. Fixing it by
using char* without converting it to std::string.

Test: run-gtests.sh
JniCompilerTest.CompileAndRunIntMethodThroughStubNormalCompiler
is successful.

Change-Id: I91249da7c1a72a2dff9bc77e477b465e0c7ee056