MINOR: quic: Possible overflow in qpack_get_varint()

This should fix CID 375051 in GH 1536 where a signed integer expression (1 << bit)
which could overflow was compared to a uint64_t.
diff --git a/src/qpack-dec.c b/src/qpack-dec.c
index 6c55495..3bfa8e9 100644
--- a/src/qpack-dec.c
+++ b/src/qpack-dec.c
@@ -67,8 +67,8 @@
 	uint8_t shift = 0;
 
 	len--;
-	ret = *raw++ & ((1 << b) - 1);
-	if (ret != (uint64_t)((1 << b) - 1))
+	ret = *raw++ & ((1ULL << b) - 1);
+	if (ret != (uint64_t)((1ULL << b) - 1))
 		goto end;
 
 	while (len && (*raw & 128)) {