[1/2] jscript: Return NULL if jsstr_substr fails.

[Jacek Caban]

Hi Sebastian,

I took a deeper look at this. It seems that we have at least two more
jsstr_alloc_buf callers to fix (jsstr_concat and
JSGlobal_decodeURIComponent). All those seem to be mistakes caused by
wrong choice of function signature, requiring not intuitive error
handling. I'd prefer to change the root of those problems, making the
function:
jsstr_t *jsstr_alloc_buf(unsigned,WCHAR**)
This should avoid similar mistakes in the future.

This will be much larger patch than you originally meant, so please let
me know if you'd like to prepare a patch or should I take care of that.

Thanks,
Jacek