ntlm: explicit type casting

lib/curl_ntlm_core.c

@@ -552,7 +552,7 @@ CURLcode Curl_ntlm_core_mk_nt_hash(struct Curl_easy *data,
   CURLcode result;
   if(len > SIZE_T_MAX/2) /* avoid integer overflow */
     return CURLE_OUT_OF_MEMORY;
-  pw = len ? malloc(len * 2) : strdup("");
+  pw = len ? malloc(len * 2) : (unsigned char *)strdup("");
   if(!pw)
     return CURLE_OUT_OF_MEMORY;