When calling OSMesaGetProcAdress("OSMesaPixelStore") the function returns a NULL value. The reason for this is that the function table in src/mesa/drivers/osmesa/osmesa.c (which is repeated in src/gallium/state_trackers/osmesa/osmesa.c) contains an additional "s" ("OSMesaPixelsStore")
Thanks. Patch posted to mesa-dev for review.
Fixed with commit 141e1eb29fe80ad341e718147a1277cc3b1b9c11