numparam/spicenum.c, stripbraces(), bug fix

overwriting SPICE_DSTRING `dstr_p' may invalidate `s'

This bug was injected with the introduction of SPICE_DSTRING in :
> commit 76feebbbfa
> Date:   Thu Feb 25 21:43:03 2010 +0000
>     various bug fixes

src/frontend/numparam/spicenum.c

@@ -149,8 +149,7 @@ stripbraces(SPICE_DSTRINGPTR dstr_p)
                 cadd(&tstr, ' ');
 
             i = spice_dstring_length(&tstr);
-            pscopy(dstr_p, s, j, ls);
-            sadd(&tstr, s);
+            sadd(&tstr, s + j);
             scopyd(dstr_p, &tstr);
             s = spice_dstring_value(dstr_p);
             ls = spice_dstring_length(dstr_p);