y4m_test: fix segfault if test files are missing
y4m_test: fix segfault if test files are missing

Change-Id: I7a04beb83095e5c0821048909f81f45be8b5eee3

shacklettbp/aom,shacklettbp/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,shacklettbp/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,jmvalin/aom,thdav/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,shacklettbp/aom,shacklettbp/aom,shacklettbp/aom,thdav/aom,luctrudeau/aom,mbebenita/aom,thdav/aom,thdav/aom,jmvalin/aom,GrokImageCompression/aom,luctrudeau/aom,smarter/aom,jmvalin/aom,mbebenita/aom,thdav/aom,GrokImageCompression/aom,jmvalin/aom,jmvalin/aom,luctrudeau/aom,jmvalin/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,thdav/aom