ltraacct.c, bug fix

reported by Gary Hennigan
on tracker
  http://sourceforge.net/tracker/index.php?func=detail&aid=3591132&group_id=38962&atid=423915

access to uninitialized memory,
  trying to acquire the "time" of a projected auxiliary timestep in the past.

now using the following auxiliary timestep t[-1]

  voltage(t[-1])  ===  voltage(t[0])
and
  t[-1]   ===   t[0]  -  (t[1] - t[0])

the details are not understood,
  but thats certainly better than accessing uninitialized memory.

src/spicelib/devices/ltra/ltraacct.c

@@ -159,6 +159,8 @@ LTRAaccept(CKTcircuit *ckt, GENmodel *inModel)
 	    return (error);
 	}
 #else
+        double dt_01, dt_12;
+
 	/*
 	 * remove the hack here - store the total inputs for the last 2 or 3
 	 * timesteps
@@ -182,18 +184,19 @@ LTRAaccept(CKTcircuit *ckt, GENmodel *inModel)
 	v6 = ckt->CKTtimeIndex < 2 ? v5 : (*(here->LTRAv2 + ckt->CKTtimeIndex - 2) +
 	    *(here->LTRAi2 + ckt->CKTtimeIndex - 2) *
 	    model->LTRAimped) * model->LTRAattenuation;
-	d1 = (v1 - v2) / (*(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex) - *(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex - 1));
-	d2 = (v2 - v3) / (*(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex - 1) - *(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex - 2));
-	d3 = (v4 - v5) / (*(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex) - *(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex - 1));
-	d4 = (v5 - v6) / (*(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex - 1) - *(ckt->CKTtimePoints +
-		ckt->CKTtimeIndex - 2));
+
+        dt_01 = (*(ckt->CKTtimePoints + ckt->CKTtimeIndex) -
+                 *(ckt->CKTtimePoints + ckt->CKTtimeIndex - 1));
+
+        dt_12 = (ckt->CKTtimeIndex < 2)
+            ? dt_01
+            : (*(ckt->CKTtimePoints + ckt->CKTtimeIndex - 1) -
+               *(ckt->CKTtimePoints + ckt->CKTtimeIndex - 2));
+
+	d1 = (v1 - v2) / dt_01;
+	d2 = (v2 - v3) / dt_12;
+	d3 = (v4 - v5) / dt_01;
+	d4 = (v5 - v6) / dt_12;
 
 	/*
 	 * here we have a big problem with the scheme boxed by the *s below.