Make type conversion explicit

This fixes a few compiler warnings for MSVC.

Change-Id: If4fa6eb3638e4cb92e982c7444e63fdc09c6f16e

av1/av1_dx_iface.c

diff --git a/av1/av1_dx_iface.c b/av1/av1_dx_iface.c
index afc52a1..b5ae871 100644
--- a/av1/av1_dx_iface.c
+++ b/av1/av1_dx_iface.c
@@ -223,7 +223,7 @@
     // Skip any associated payload (there shouldn't be one, but just in case)
     if (data_sz < bytes_read + payload_size) return AOM_CODEC_CORRUPT_FRAME;
     data += bytes_read + payload_size;
-    data_sz -= bytes_read + payload_size;
+    data_sz -= (uint32_t)(bytes_read + payload_size);
 
     status = aom_read_obu_header_and_size(
         data, data_sz, si->is_annexb, &obu_header, &payload_size, &bytes_read);
@@ -235,7 +235,7 @@
 
   // Read a few values from the sequence header payload
   data += bytes_read;
-  data_sz -= bytes_read;
+  data_sz -= (uint32_t)bytes_read;
   struct aom_read_bit_buffer rb = { data, data + data_sz, 0, NULL, NULL };
 
   av1_read_profile(&rb);  // profile