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 iits args on
the top-level Context which it set with all strings.  Changed
calldatafree() to set pointers to NULL after it frees them.
Merge pull request #1655 from tshinnic/script_native_AddXHint_width

Native script AddXHint width arg fix; error double free fix