Use an alternative fix to ubsan warning.

This commit revert the previous fix of the ubsan warning for unsigned
int overflow, and use a better fix by  moving the offs-- inside the
while loop to avoid "0-1" situation.

Change-Id: Id4a3e03859ebcdf264df0808412b30841028f87c

aom_dsp/entenc.c

@@ -514,7 +514,7 @@ unsigned char *od_ec_enc_done(od_ec_enc *enc, uint32_t *nbytes) {
   unsigned char *out;
   uint32_t storage;
   uint16_t *buf;
-  int offs;
+  uint32_t offs;
   uint32_t end_offs;
   int nend_bits;
   od_ec_window m;
@@ -554,7 +554,7 @@ unsigned char *od_ec_enc_done(od_ec_enc *enc, uint32_t *nbytes) {
   if (s > 0) {
     unsigned n;
     storage = enc->precarry_storage;
-    if (offs + ((s + 7) >> 3) > (int)storage) {
+    if (offs + ((s + 7) >> 3) > storage) {
       storage = storage * 2 + ((s + 7) >> 3);
       buf = (uint16_t *)realloc(buf, sizeof(*buf) * storage);
       if (buf == NULL) {
@@ -566,7 +566,7 @@ unsigned char *od_ec_enc_done(od_ec_enc *enc, uint32_t *nbytes) {
     }
     n = (1 << (c + 16)) - 1;
     do {
-      OD_ASSERT(offs < (int)storage);
+      OD_ASSERT(offs < storage);
       buf[offs++] = (uint16_t)(e >> (c + 16));
       e &= n;
       s -= 8;
@@ -607,7 +607,8 @@ unsigned char *od_ec_enc_done(od_ec_enc *enc, uint32_t *nbytes) {
   out = out + storage - (offs + end_offs);
   c = 0;
   end_offs = offs;
-  while (offs-- > 0) {
+  while (offs > 0) {
+    offs--;
     c = buf[offs] + c;
     out[offs] = (unsigned char)c;
     c >>= 8;