Fixed a potential null pointer dereference and a case of undefined behaviour

src/common/StringMap.h

@@ -126,7 +126,7 @@ public:
 
 		if (index >= SIZE)
 		{
-			printf("\nConstant %s out of bounds with %i!\n", key, index);
+			printf("Constant %s out of bounds with %u!\n", key, index);
 			return false;
 		}
 

src/common/runtime.cpp

@@ -538,7 +538,7 @@ int luax_insist(lua_State *L, int idx, const char *k)
 {
 	// Convert to absolute index if necessary.
 	if (idx < 0 && idx > LUA_REGISTRYINDEX)
-		idx = lua_gettop(L) + ++idx;
+		idx += lua_gettop(L) + 1;
 
 	lua_getfield(L, idx, k);