Change exit(0) to assert(0) in txb_common.h
Change exit(0) to assert(0) in txb_common.h

This will provide better indication of abnormal code flow.

Change-Id: I7e6cdcf77f13c709137e6524c94b0911cd5216ed

smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,GrokImageCompression/aom