fix potential integer overflow in raw API (#103)

Author: dloebl

src/cgif_raw.c

@@ -330,7 +330,7 @@ static int LZW_GenerateStream(LZWResult* pResult, const uint32_t numPixel, const
   // where N = max dictionary resets = numPixel / (MAX_DICT_LEN - initDictLen - 2)
   entriesPerCycle = MAX_DICT_LEN - initDictLen - 2; // maximum added number of dictionary entries per cycle: -2 to account for start and end code
   maxResets = numPixel / entriesPerCycle;
-  pContext->pLZWData   = malloc(sizeof(uint16_t) * (numPixel + 2 + maxResets));
+  pContext->pLZWData   = malloc(sizeof(uint16_t) * ((size_t)numPixel + 2 + maxResets));
   if(pContext->pLZWData == NULL) {
     r = CGIF_EALLOC;
     goto LZWGENERATE_Cleanup;