spicelib/parser/inpptree.c, bug fix, "#287 Shared library crashes on using non existing functions "

"remcirc" has the potential to invoke INPfreeTree() with a NULL IFparseTree

Thanks to Craggan, who reported this in:
  http://sourceforge.net/p/ngspice/bugs/287/

src/spicelib/parser/inpptree.c

@@ -1409,6 +1409,9 @@ void INPfreeTree(IFparseTree *ptree)
 
     int i;
 
+    if (!pt)
+        return;
+
     for (i = 0; i < pt->p.numVars; i++)
         dec_usage(pt->derivs[i]);