patch 8.2.0143: Coverity warning for possible use of NULL pointer

Problem:    Coverity warning for possible use of NULL pointer.
Solution:   Check argv is not NULL.
diff --git a/src/channel.c b/src/channel.c
index cd791b5..3aec7c8 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -5993,7 +5993,7 @@
 #ifndef USE_ARGV
     vim_free(ga.ga_data);
 #endif
-    if (argv != job->jv_argv)
+    if (argv != NULL && argv != job->jv_argv)
     {
 	for (i = 0; argv[i] != NULL; i++)
 	    vim_free(argv[i]);