lib:vector:vlib: Fix possible null pointer dereference

In the function `Vect_cat_list_to_array`, as part of the
execution, if list turns out to not contain any numbers,
`cats` internal variable is not changed from NULL. Without
checking if `cats` is NULL or not, qsort or first elemnt of
it is accessed, which can lead to null pointer dereference.

To fix that issue, only access cats if it's not NULL.

This issue was found using cppcheck tool.

Signed-off-by: Mohan Yelugoti <[email protected]>

lib/vector/Vlib/cats.c

@@ -493,7 +493,7 @@ int Vect_cat_list_to_array(const struct cat_list *list, int **vals, int *nvals)
 
     G_debug(1, "Vect_cat_list_to_array()");
 
-    *nvals = n_cats = 0;
+    *nvals = n_cats = n_ucats = 0;
     cats = NULL;
     for (i = 0; i < list->n_ranges; i++) {
         n = list->max[i] - list->min[i] + 1;
@@ -509,13 +509,16 @@ int Vect_cat_list_to_array(const struct cat_list *list, int **vals, int *nvals)
         n_cats += n;
     }
 
-    /* sort array */
-    qsort(cats, n_cats, sizeof(int), cmp);
+    if (cats) {
+        /* sort array */
+        qsort(cats, n_cats, sizeof(int), cmp);
+
+        /* skip duplicated values */
+        ucats = G_malloc(sizeof(int) * n_cats);
+        last_cat = ucats[0] = cats[0];
+        n_ucats = 1;
+    }
 
-    /* skip duplicated values */
-    ucats = G_malloc(sizeof(int) * n_cats);
-    last_cat = ucats[0] = cats[0];
-    n_ucats = 1;
     for (i = 1; i < n_cats; i++) {
         if (last_cat == cats[i])
             continue;