Fix two crashes caused by trying to plot after circuit parsing failed.

src/spicelib/analysis/cktfndm.c

@@ -16,5 +16,5 @@ Author: 1985 Thomas L. Quarles
 GENmodel *
 CKTfndMod(CKTcircuit *ckt, IFuid modname)
 {
-    return nghash_find(ckt->MODnameHash, modname);
+    return ckt ? nghash_find(ckt->MODnameHash, modname) : NULL;
 }

src/spicelib/devices/cktfinddev.c

@@ -13,5 +13,5 @@ Author: 1985 Thomas L. Quarles
 GENinstance *
 CKTfndDev(CKTcircuit *ckt, IFuid name)
 {
-    return nghash_find(ckt->DEVnameHash, name);
+    return ckt ? nghash_find(ckt->DEVnameHash, name) : NULL;
 }