BUILD: http_fetch: silence an uninitiialized warning with gcc-4/5/6 at -Os

Gcc 4.x, 5.x and 6.x report this when compiling http_fetch.c:

  src/http_fetch.c: In function 'smp_fetch_meth':
  src/http_fetch.c:357:6: warning: 'htx' may be used uninitialized in this function [-Wmaybe-uninitialized]
     sl = http_get_stline(htx);

That's quite weird since there's no such code path, but presetting the
htx variable to NULL during declaration is enough to shut it up.

This may be backported to any version that has dbbdb25f1 ("BUG/MINOR:
http-fetch: Use integer value when possible in "method" sample fetch")
as it's the one that triggered this warning (hence at least 2.0).

src/http_fetch.c

@@ -334,7 +334,7 @@ static int smp_fetch_meth(const struct arg *args, struct sample *smp, const char
 {
 	struct channel *chn = SMP_REQ_CHN(smp);
 	struct http_txn *txn;
-	struct htx *htx;
+	struct htx *htx = NULL;
 	int meth;
 
 	txn = (smp->strm ? smp->strm->txn : NULL);