security: silence conversion warning

With MinGW-w64, `curl_socket_t` is is a 32 or 64 bit unsigned integer,
while `read` expects a 32 bit signed integer.
Use `sread` instead of `read` to use the correct parameter type.

Closes https://github.com/curl/curl/pull/4483

lib/security.c

@@ -236,7 +236,7 @@ static ssize_t sec_recv(struct connectdata *conn, int sockindex,
 
   /* Handle clear text response. */
   if(conn->sec_complete == 0 || conn->data_prot == PROT_CLEAR)
-      return read(fd, buffer, len);
+      return sread(fd, buffer, len);
 
   if(conn->in_buffer.eof_flag) {
     conn->in_buffer.eof_flag = 0;