util: Allow Assert(...) to be used in all contexts

Fixes the compile error when used inside operator[]:

./chain.h:404:23: error: C++11 only allows consecutive left square brackets when introducing an attribute
        return (*this)[Assert(pindex)->nHeight] == pindex;
                      ^

src/util/check.h

@@ -54,6 +54,6 @@ T get_pure_r_value(T&& val)
 }
 
 /** Identity function. Abort if the value compares equal to zero */
-#define Assert(val) [&]() -> decltype(get_pure_r_value(val)) { auto&& check = (val); assert(#val && check); return std::forward<decltype(get_pure_r_value(val))>(check); }()
+#define Assert(val) ([&]() -> decltype(get_pure_r_value(val)) { auto&& check = (val); assert(#val && check); return std::forward<decltype(get_pure_r_value(val))>(check); }())
 
 #endif // BITCOIN_UTIL_CHECK_H