[CLEANUP] remove a warning from gcc due to htons() in standard.c

[CLEANUP] remove a warning from gcc due to htons() in standard.c

Due to the fact that htons is defined as a macro, it's dangerous
to call it with auto-incremented arguments such as htons(f(++x)) :

src/standard.c: In function 'url2sa':
src/standard.c:291: warning: operation on 'curr' may be undefined

The solution is simply to store the intermediate result an pass it
to htons() at once.