CLEANUP: hpack: remove a redundant test in the decoder

As reported in issue #485 the test for !len at the end of the
loop in get_var_int() is useless since it was already done inside
the loop. Actually the code is more readable if we remove the first
one so let's do this instead. The resulting code is exactly the same
since the compiler already optimized the test away.
diff --git a/src/hpack-dec.c b/src/hpack-dec.c
index 7f3e762..a721fad 100644
--- a/src/hpack-dec.c
+++ b/src/hpack-dec.c
@@ -65,11 +65,7 @@
 	if (ret != (uint32_t)((1 << b) - 1))
 		goto end;
 
-	while (1) {
-		if (!len)
-			goto too_short;
-		if (!(*raw & 128))
-			break;
+	while (len && (*raw & 128)) {
 		ret += ((uint32_t)(*raw++) & 127) << shift;
 		shift += 7;
 		len--;