v.out.ogr: Check for valid array before passing to qsort (OSGeo#4278)

Currently, filling list with GDAL driver names is based on
a conditional and if the conditional fails, the list can be NULL.

We pass this list to qsort to sort the names, but behavior
of qsort with a NULL array is undefined. To avoid getting into this
situation, check if the array is NULL before performing qsort on it.

This issue was found using cppcheck tool.

Signed-off-by: Mohan Yelugoti <ymdatta.work@gmail.com>

vector/v.out.ogr/list.c

@@ -43,7 +43,8 @@ char *OGR_list_write_drivers(void)
         len += strlen(buf) + 1; /* + ',' */
     }
 
-    qsort(list, count, sizeof(char *), cmp);
+    if (list)
+        qsort(list, count, sizeof(char *), cmp);
 
     if (len > 0) {
         ret = G_malloc((len + 1) * sizeof(char)); /* \0 */