Fix a bug in vpxenc reading raw input frame

The bug was introduced in 00a35aab. The reading of raw yuv input frame
was off by 4 bytes.

Change-Id: I6923ea5528aa529a47a06b64adca8f94847f19a6

ivfdec.c

diff --git a/ivfdec.c b/ivfdec.c
index 51f4a09..4a0816f 100644
--- a/ivfdec.c
+++ b/ivfdec.c
@@ -61,11 +61,12 @@
     }
   }
 
-  if (!is_ivf)
+  if (!is_ivf) {
     rewind(input_ctx->file);
-  else
+    input_ctx->detect.buf_read = 0;
+  } else {
     input_ctx->detect.position = 4;
-
+  }
   return is_ivf;
 }