inpptree parser, bug fix, segfault due to usage of an unknown function

for example:
B1  1 0  V = {log(2.0, 1.3) + nosuchthing(2.0, 1.3)}

src/spicelib/parser/inpptree-parser.y

@@ -106,7 +106,10 @@ exp:
 
   | '-' exp  %prec NEG                { $$ = mkfnode("-",$2); }
 
-  | TOK_STR '(' nonempty_arglist ')'  { $$ = mkfnode($1, $3); txfree((void*)$1); }
+  | TOK_STR '(' nonempty_arglist ')'  { $$ = mkfnode($1, $3);
+                                        if (!$$)
+                                            YYERROR;
+                                        txfree((void*)$1); }
 
   | TOK_pnode
 

src/spicelib/parser/inpptree.c

@@ -197,7 +197,7 @@ static struct constant {
 void
 INPgetTree(char **line, INPparseTree ** pt, CKTcircuit *ckt, INPtables * tab)
 {
-    INPparseNode *p;
+    INPparseNode *p = NULL;
     int i, rv;
 
     values = NULL;