[packager] Fix Potential segfault crash

modules/fxpackager/src/main/native/library/common/Package.cpp: assert(FBootFields != NULL);

can cause a segfault or null point exception. This should be fixed to handle the error case better.

Package.cpp:

void Package::FreeBootFields() {
    if (FBootFields != NULL) {
        delete FBootFields;
        FBootFields = NULL;
    }
}

JavaVirtualMachine.cpp:

    // On Mac we can only free the boot fields if the calling thread is not the main thread.
    #ifdef MAC
    if (platform.IsMainThread() == false) {
        package.FreeBootFields();
    }
    #else
    package.FreeBootFields();
    #endif //MAC