add function trnoise_state_free and use it

instead of its individual components

src/frontend/trannoise/1-f-code.c

@@ -241,3 +241,12 @@ trrandom_state_init(int rndtype, double TS, double TD, double PARAM1, double PAR
 
     return this;
 }
+
+void
+trnoise_state_free(struct trnoise_state *this)
+{
+    if (!this)
+        return;
+    tfree(this->oneof);
+    tfree(this);
+}

src/spicelib/devices/vsrc/vsrcdel.c

@@ -16,10 +16,7 @@ VSRCdelete(GENinstance *gen_inst)
     VSRCinstance *inst = (VSRCinstance *) gen_inst;
 
     FREE(inst->VSRCcoeffs);
-    if (inst->VSRCtrnoise_state) {
-        FREE(inst->VSRCtrnoise_state->oneof);
-        FREE(inst->VSRCtrnoise_state);
-    }
+    trnoise_state_free(inst->VSRCtrnoise_state);
     FREE(inst->VSRCtrrandom_state);
 
     return OK;