Add a selftest for detecting incorrect packed duk_tval guess
Add a selftest for detecting incorrect packed duk_tval guess

Without this selftest, an incorrect guess leads to crashes (see e.g.
GH-15).

haosu1987/duktape,sloth4413/duktape,tassmjau/duktape,zeropool/duktape,harold-b/duktape,haosu1987/duktape,eddieh/duktape,skomski/duktape,haosu1987/duktape,daimajia/duktape,tassmjau/duktape,eddieh/duktape,skomski/duktape,harold-b/duktape,sloth4413/duktape,kphillisjr/duktape,thurday/duktape,pombredanne/duktape,chenyaqiuqiu/duktape,thurday/duktape,svaarala/duktape,eddieh/duktape,rwaldron/duktape,markand/duktape,jmptrader/duktape,kphillisjr/duktape,eddieh/duktape,harold-b/duktape,chenyaqiuqiu/duktape,skomski/duktape,skomski/duktape,tassmjau/duktape,markand/duktape,skomski/duktape,reqshark/duktape,daimajia/duktape,pombredanne/duktape,kphillisjr/duktape,tassmjau/duktape,jmptrader/duktape,haosu1987/duktape,harold-b/duktape,zeropool/duktape,daimajia/duktape,kphillisjr/duktape,reqshark/duktape,nivertech/duktape,jmptrader/duktape,reqshark/duktape,harold-b/duktape,harold-b/duktape,haosu1987/duktape,tassmjau/duktape,kphillisjr/duktape,haosu1987/duktape,harold-b/duktape,markand/duktape,tassmjau/duktape,thurday/duktape,skomski/duktape,pombredanne/duktape,chenyaqiuqiu/duktape,reqshark/duktape,nivertech/duktape,reqshark/duktape,daimajia/duktape,rwaldron/duktape,kphillisjr/duktape,haosu1987/duktape,pombredanne/duktape,harold-b/duktape,thurday/duktape,thurday/duktape,nivertech/duktape,markand/duktape,pombredanne/duktape,markand/duktape,chenyaqiuqiu/duktape,sloth4413/duktape,svaarala/duktape,markand/duktape,jmptrader/duktape,jmptrader/duktape,haosu1987/duktape,pombredanne/duktape,harold-b/duktape,kphillisjr/duktape,eddieh/duktape,tassmjau/duktape,markand/duktape,svaarala/duktape,zeropool/duktape,sloth4413/duktape,markand/duktape,chenyaqiuqiu/duktape,thurday/duktape,markand/duktape,sloth4413/duktape,eddieh/duktape,rwaldron/duktape,chenyaqiuqiu/duktape,eddieh/duktape,zeropool/duktape,jmptrader/duktape,sloth4413/duktape,zeropool/duktape,kphillisjr/duktape,jmptrader/duktape,nivertech/duktape,reqshark/duktape,chenyaqiuqiu/duktape,svaarala/duktape,thurday/duktape,tassmjau/duktape,pombredanne/duktape,jmptrader/duktape,skomski/duktape,sloth4413/duktape,skomski/duktape,chenyaqiuqiu/duktape,reqshark/duktape,zeropool/duktape,zeropool/duktape,tassmjau/duktape,rwaldron/duktape,jmptrader/duktape,svaarala/duktape,nivertech/duktape,markand/duktape,reqshark/duktape,haosu1987/duktape,nivertech/duktape,thurday/duktape,kphillisjr/duktape,svaarala/duktape,harold-b/duktape,kphillisjr/duktape,skomski/duktape,reqshark/duktape,reqshark/duktape,thurday/duktape,daimajia/duktape,pombredanne/duktape,eddieh/duktape,sloth4413/duktape,skomski/duktape,pombredanne/duktape,chenyaqiuqiu/duktape,svaarala/duktape,eddieh/duktape,nivertech/duktape,svaarala/duktape,nivertech/duktape,haosu1987/duktape,zeropool/duktape,pombredanne/duktape,sloth4413/duktape,rwaldron/duktape,tassmjau/duktape,rwaldron/duktape,jmptrader/duktape,chenyaqiuqiu/duktape,daimajia/duktape,eddieh/duktape,sloth4413/duktape,nivertech/duktape,zeropool/duktape,zeropool/duktape,nivertech/duktape,svaarala/duktape,thurday/duktape