lua/bytevarlib: return luaL_error to suppress warning

Even though luaL_error never returns, use a return to make it
clear. Also prevents a compiler warning about idx being used
uninitialized.

This is a compiler warning with LTO.

Author: jasonish

src/util-lua-bytevarlib.c

@@ -56,7 +56,7 @@ static int LuaBytevarMap(lua_State *L)
 
     DetectByteIndexType idx;
     if (!DetectByteRetrieveSMVar(name, s, -1, &idx)) {
-        luaL_error(L, "unknown byte_extract or byte_math variable: %s", name);
+        return luaL_error(L, "unknown byte_extract or byte_math variable: %s", name);
     }
 
     ld->bytevar[ld->bytevars].name = SCStrdup(name);