FS-8413: Segfault calling session:getVariable(nil) in lua script

script calling session:getVariable() with a null variable
name will cause FreeSWITCH to segfault.

This change checks whether varname parameter to
switch_channel_get_variable_dup is non-NULL.

src/switch_channel.c

@@ -930,6 +930,7 @@ SWITCH_DECLARE(const char *) switch_channel_get_variable_dup(switch_channel_t *c
 
 	switch_mutex_lock(channel->profile_mutex);
 
+	if (!zstr(varname)) {
 		if (channel->scope_variables) {
 			switch_event_t *ep;
 
@@ -959,6 +960,7 @@ SWITCH_DECLARE(const char *) switch_channel_get_variable_dup(switch_channel_t *c
 				}
 			}
 		}
+	}
 
 	if (dup && v != vdup) {
 		if (v) {