Native script AddXHint width arg fix; error double free fix

Coverity pointed out that `fontforge/scripting.c` :: `_AddHint()` was missing
setting variable `width` in one place.  This looks to be a simple typo.

Testing before fix generated error message "AddHHint: Bad hint width"
as the uninitialized value of `width` was -18032.  Testing after the
fix saw no error message.  The line of code was:
>       AddHHint(542.5,10.25);

While testing under gdb a double free was detected while cleaning up
after the native script error was declared.  Call path
>      ScriptError()->traceback()->calldatafree()

was freeing all string arguments in all `Context`s on the stack.  But
then `ProcessNativeScript()` at end *also* wanted to free its args on
the top-level `Context` which it set with all strings.  Changed
`calldatafree()` to set pointers to NULL after it frees them.