v.external: Check for valid list before passing to qsort (OSGeo#4280)

Currently, if 'HAVE_OGR' macro is defined, as part of execution,
we order all formats by name using qsort. But, list containing
all formats is assigned based on a conditional and if the
conditional fails, it can be NULL.
Behavior of qsort is undefined when a NULL ptr is passed as array
argument. To avoid getting into that 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.external/list.c

@@ -64,7 +64,8 @@ char **format_list(int *count, size_t *len)
     }
 
     /* order formats by name */
-    qsort(list, *count, sizeof(char *), cmp);
+    if (list)
+        qsort(list, *count, sizeof(char *), cmp);
 #endif
 #if defined HAVE_POSTGRES && !defined HAVE_OGR
     list = G_realloc(list, ((*count) + 1) * sizeof(char *));