Made descriptions of invalid interpolation cases more descriptive and fixed a memory leak

src/frontend/interp.c

@@ -17,37 +17,45 @@ Author: 1985 Wayne A. Christopher, U. C. Berkeley CAD Group
 void
 lincopy(struct dvec *ov, double *newscale, int newlen, struct dvec *oldscale)
 {
-    struct dvec *v;
-    double *nd;
-
     if (!isreal(ov)) {
-        fprintf(cp_err, "Warning: vector %s is not real - no interpolation\n", ov->v_name);
+        fprintf(cp_err, "Warning: vector %s is a complex vector - "
+                "complex vectors cannot be interpolated\n",
+                ov->v_name);
         return;
     }
 
     if (ov->v_length == 1) {
-        fprintf(cp_err, "Warning: %s is not a vector - no interpolation\n", ov->v_name);
+        fprintf(cp_err, "Warning: %s is a scalar - "
+                "interpolation is not possible\n",
+                ov->v_name);
         return;
     }
 
     if (ov->v_length < oldscale->v_length) {
-        fprintf(cp_err, "Warning: vector %s is too short - no interpolation\n", ov->v_name);
+        fprintf(cp_err, "Warning: %s only contains %d points - "
+                "interpolation is not performed unless there are "
+                "at least as many points as the scale vector (%d)\n",
+                ov->v_name, ov->v_length, oldscale->v_length);
         return;
     }
 
-    v = dvec_alloc(copy(ov->v_name),
+    /* Allocate the vector to receive the linearized data */
+    struct dvec * const v = dvec_alloc(copy(ov->v_name),
                    ov->v_type,
                    ov->v_flags | VF_PERMANENT,
                    newlen, NULL);
 
-    nd = v->v_realdata;
-    if (!ft_interpolate(ov->v_realdata, nd, oldscale->v_realdata,
-                        oldscale->v_length, newscale, newlen, 1))
-    {
+    /* Do interpolation and then add the vector to the current plot. If
+     * interpolation fails, the vector must be freed. */
+    if (!ft_interpolate(ov->v_realdata, v->v_realdata, oldscale->v_realdata,
+            oldscale->v_length, newscale, newlen, 1)) {
         fprintf(cp_err, "Error: can't interpolate %s\n", ov->v_name);
+        dvec_free(v);
         return;
     }
 
     vec_new(v);
-}
+} /* end of function lincopy */
+
+