Possible assert with new string semantics

[sainan@...]

The latest commit 9b4f39ab14fb2e55345c3d23537d129dac23b091 introduces 
undefined behaviour in internshrstr when LUAI_ASSERT is defined. The 
problem lies in this line of code:

memcpy(getshrstr(ts), str, l * sizeof(char));

Because shrlen is not initialised, it is possible that its value is 
0xFF, tripping the assert. This can be fixed by initialising this field 
in createstrobj or moving the line that initialises it above the memcpy.