Fix negative shift exponent in get_poc_diff

As indicated in the spec, get_poc_diff should return 0 when
order_hints are disabled.

Closes #77

src/env.h

@@ -293,6 +293,7 @@ static inline int get_comp_dir_ctx(const BlockContext *const a,
 static inline int get_poc_diff(const int order_hint_n_bits,
                                const int poc0, const int poc1)
 {
+    if (!order_hint_n_bits) return 0;
     const int mask = 1 << (order_hint_n_bits - 1);
     const int diff = poc0 - poc1;
     return (diff & (mask - 1)) - (diff & mask);