aomenc: initalize the image object

Otherwise it would cause problem when calling aom_img_free() at the end
if no frame is read.

BUG=aomedia:2911

Change-Id: I4350d5294706d2d84341e601e9ed6063229d0451

apps/aomenc.c

diff --git a/apps/aomenc.c b/apps/aomenc.c
index 789cd60..f69ffd0 100644
--- a/apps/aomenc.c
+++ b/apps/aomenc.c
@@ -1715,6 +1715,7 @@
   int profile_updated = 0;
 
   memset(&input, 0, sizeof(input));
+  memset(&raw, 0, sizeof(raw));
   exec_name = argv_[0];
 
   /* Setup default input stream settings */
@@ -1989,14 +1990,10 @@
     }
 
     if (pass == (global.pass ? global.pass - 1 : 0)) {
-      if (input.file_type == FILE_TYPE_Y4M)
-        /*The Y4M reader does its own allocation.
-          Just initialize this here to avoid problems if we never read any
-          frames.*/
-        memset(&raw, 0, sizeof(raw));
-      else
+      // The Y4M reader does its own allocation.
+      if (input.file_type != FILE_TYPE_Y4M) {
         aom_img_alloc(&raw, input.fmt, input.width, input.height, 32);
-
+      }
       FOREACH_STREAM(stream, streams) {
         stream->rate_hist =
             init_rate_histogram(&stream->config.cfg, &global.framerate);