fix bug in gl_str() when called with invalid enum
fix bug in gl_str() when called with invalid enum

RobertoMalatesta/glshim,RobertoMalatesta/glshim,lunixbochs/glshim,lunixbochs/glshim,seandepagnier/glshim,RobertoMalatesta/glshim,lunixbochs/glshim,seandepagnier/glshim,lunixbochs/glshim,seandepagnier/glshim,RobertoMalatesta/glshim,seandepagnier/glshim