Avoids MSVC warnings about possible value truncation.

nss/lib/freebl/poly1305/poly1305.c

 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /* This implementation of poly1305 is by Andrew Moon
  * (https://github.com/floodyberry/poly1305-donna) and released as public
  * domain. */
 
 #include <string.h>
 #include <stdint.h>
@@ skipping 229 matching lines @@
         state->h1 = (state->h1 & nb) | (g1 & b);
         state->h2 = (state->h2 & nb) | (g2 & b);
         state->h3 = (state->h3 & nb) | (g3 & b);
         state->h4 = (state->h4 & nb) | (g4 & b);
 
         f0 = ((state->h0      ) | (state->h1 << 26)) + (uint64_t)U8TO32_LE(&state->key[0]);
         f1 = ((state->h1 >>  6) | (state->h2 << 20)) + (uint64_t)U8TO32_LE(&state->key[4]);
         f2 = ((state->h2 >> 12) | (state->h3 << 14)) + (uint64_t)U8TO32_LE(&state->key[8]);
         f3 = ((state->h3 >> 18) | (state->h4 <<  8)) + (uint64_t)U8TO32_LE(&state->key[12]);
 
-        U32TO8_LE(&mac[ 0], f0); f1 += (f0 >> 32);
-        U32TO8_LE(&mac[ 4], f1); f2 += (f1 >> 32);
-        U32TO8_LE(&mac[ 8], f2); f3 += (f2 >> 32);
-        U32TO8_LE(&mac[12], f3);
+        U32TO8_LE(&mac[ 0], (uint32_t)f0); f1 += (f0 >> 32);
+        U32TO8_LE(&mac[ 4], (uint32_t)f1); f2 += (f1 >> 32);
+        U32TO8_LE(&mac[ 8], (uint32_t)f2); f3 += (f2 >> 32);
+        U32TO8_LE(&mac[12], (uint32_t)f3);
 }