Prevent crash upon buggy user input

src/spicelib/parser/inpptree.c

@@ -753,7 +753,7 @@ static INPparseNode *PTdifferentiate(INPparseNode * p, int varnum)
         break;
 
     default:
-        fprintf(stderr, "Internal error: bad node type %d\n", p->type);
+        fprintf(stderr, "Error: bad node type %d\n", p->type);
         newp = NULL;
         break;
     }
@@ -778,6 +778,10 @@ static INPparseNode *mkb(int type, INPparseNode * left,
     INPparseNode *p;
     int i;
 
+    if (!right || !left) {
+        return (NULL);
+    }
+
     if ((right->type == PT_CONSTANT) && (left->type == PT_CONSTANT)) {
         double value;
         switch (type) {
@@ -890,6 +894,9 @@ static INPparseNode *mkf(int type, INPparseNode * arg)
     INPparseNode *p;
     int i;
 
+    if (!arg)
+        return NULL;
+
     for (i = 0; i < NUM_FUNCS; i++)
         if (funcs[i].number == type)
             break;