pkg/lua: detect if weak symbol has been overriden.
pkg/lua: detect if weak symbol has been overriden.

After removing the default definitions of lua module tables and table
lengths (see lua_builtin.h) the symbols have been left undefined, which
results in them getting an address of NULL and a crash if there are no
user symbols and the user attempts a "require".

This patch checks the address of the table length variable and fails the
module search function of the table is not set (i.e. it behaves as if the
table was empty.)

josephnoir/RIOT,rfuentess/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,smlng/RIOT,x3ro/RIOT,yogo1212/RIOT,OTAkeys/RIOT,josephnoir/RIOT,ant9000/RIOT,aeneby/RIOT,x3ro/RIOT,rfuentess/RIOT,toonst/RIOT,kYc0o/RIOT,jasonatran/RIOT,yogo1212/RIOT,mtausig/RIOT,mfrey/RIOT,miri64/RIOT,miri64/RIOT,authmillenon/RIOT,aeneby/RIOT,OlegHahm/RIOT,aeneby/RIOT,basilfx/RIOT,miri64/RIOT,mtausig/RIOT,miri64/RIOT,basilfx/RIOT,cladmi/RIOT,josephnoir/RIOT,OlegHahm/RIOT,toonst/RIOT,authmillenon/RIOT,ant9000/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,cladmi/RIOT,smlng/RIOT,mtausig/RIOT,kaspar030/RIOT,josephnoir/RIOT,mfrey/RIOT,mtausig/RIOT,miri64/RIOT,ant9000/RIOT,cladmi/RIOT,authmillenon/RIOT,josephnoir/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,toonst/RIOT,rfuentess/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,basilfx/RIOT,authmillenon/RIOT,jasonatran/RIOT,toonst/RIOT,mfrey/RIOT,yogo1212/RIOT,smlng/RIOT,OlegHahm/RIOT,yogo1212/RIOT,kYc0o/RIOT,x3ro/RIOT,smlng/RIOT,basilfx/RIOT,authmillenon/RIOT,mfrey/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,toonst/RIOT,smlng/RIOT,rfuentess/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,x3ro/RIOT,rfuentess/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OlegHahm/RIOT,cladmi/RIOT,mtausig/RIOT,RIOT-OS/RIOT,aeneby/RIOT,x3ro/RIOT,jasonatran/RIOT,cladmi/RIOT,yogo1212/RIOT,kYc0o/RIOT,mfrey/RIOT,aeneby/RIOT