Remove two unnecessary/incorrect uint32_t casts

In av1_write_uleb_obu_size(), it is incorrect to cast obu_payload_size
to uint32_t. If obu_payload_size is greater than UINT32_MAX, the
uint32_t cast will truncate its value. Pass obu_payload_size to
aom_uleb_encode() and it will return an error if obu_payload_size is
greater than UINT32_MAX.

In av1_write_metadata_obu(), it is not necessary to cast the return
value to uint32_t.

Change-Id: Ida23bfe50da5841c5331978d75c4fee1ec61843a

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index d0d1820..44cb43b 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3405,10 +3405,8 @@
                             uint8_t *dest) {
   const size_t offset = obu_header_size;
   size_t coded_obu_size = 0;
-  const uint32_t obu_size = (uint32_t)obu_payload_size;
-  assert(obu_size == obu_payload_size);
 
-  if (aom_uleb_encode(obu_size, sizeof(obu_size), dest + offset,
+  if (aom_uleb_encode(obu_payload_size, sizeof(uint32_t), dest + offset,
                       &coded_obu_size) != 0) {
     return AOM_CODEC_ERROR;
   }
@@ -4144,7 +4142,7 @@
   memcpy(dst + coded_metadata_size, metadata->payload, metadata->sz);
   // Add trailing bits.
   dst[coded_metadata_size + metadata->sz] = 0x80;
-  return (uint32_t)(coded_metadata_size + metadata->sz + 1);
+  return coded_metadata_size + metadata->sz + 1;
 }
 
 static size_t av1_write_metadata_array(AV1_COMP *const cpi, uint8_t *dst) {