fix a bug: re-enable reading vecs with length 1

Bug reported by LM

src/sharedspice.c

@@ -1884,7 +1884,7 @@ int sh_ExecutePerLoop(void)
     /* get the data of the last entry to the plot vector */
     veclen = pl->pl_dvecs->v_length - 1;
     /* safeguard against vectors with 0 length (e.g. @c1[i] during ac simulation) */
-    if (veclen < 1)
+    if (veclen < 0)
         return 2;
     curvecvalsall->vecindex = veclen;
     for (d = pl->pl_dvecs, i = 0; d; d = d->v_next, i++) {