Fix divide by zero in TextureGLRL.py

File: TextureGLRL.py
Lines: 112-126
Error: Division by zero
Result: NAN value for GLRL features
Fix: include a check for sum_P[theta] equal to zero
Notes:  It is possible for sum_P[theta] to be zero in the event that the GLRL matrix contains no gray-level runs
in the direction theta.  This problem arose a number of times while processing a GBM dataset.