Fix crash when minipbrt::Loader::load(filename) could not open the file provided and crash with loop subdiv

[Uedaki]

Fix 1

When failing to open a file (minipbrt.cpp:4676), minipbrt would call set_error() prior to the creation m_filedata allocated few lines after. This would create a crash when set_error() is trying to access the pointer to retrieve the filename and the error offset.

Fix 2

I encountered a crash when trying to triangulate a loop subdiv. It seems that the number of points was not divided by 3 after the call to float_vector_param generating a crash inside the LoopSubdiv::triangle_mesh() function which is trying to copy num_points * 3 floats from float P[num_points] to float trimesh->P[num_points * 3]. I fixed the issue by dividing the number of points by 3 when creating a loop subdiv.