Abort if firstpass file does not exist

This fixes a crash if the firstpass file does not
exist when doing a two-pass encode.

Change-Id: I3a1a95d68d57125c63123d6208af7537f5a689a0

vpxstats.c

@@ -41,6 +41,9 @@ int stats_open_file(stats_io_t *stats, const char *fpf, int pass) {
 
     stats->file = fopen(fpf, "rb");
 
+    if (stats->file == NULL)
+      fatal("First-pass stats file does not exist!");
+
     if (fseek(stats->file, 0, SEEK_END))
       fatal("First-pass stats file must be seekable!");