only enable passthru_ptime_mismatch when set to a true value, not when it is set to any value

src/switch_channel.c

@@ -3376,7 +3376,7 @@ SWITCH_DECLARE(switch_status_t) switch_channel_perform_mark_pre_answered(switch_
 		switch_channel_api_on(channel, SWITCH_CHANNEL_API_ON_PRE_ANSWER_VARIABLE);
 		switch_channel_api_on(channel, SWITCH_CHANNEL_API_ON_MEDIA_VARIABLE);
 
-		if ((var = switch_channel_get_variable(channel, SWITCH_PASSTHRU_PTIME_MISMATCH_VARIABLE))) {
+		if ((var = switch_channel_get_variable(channel, SWITCH_PASSTHRU_PTIME_MISMATCH_VARIABLE)) && switch_true(var)) {
 			switch_channel_set_flag(channel, CF_PASSTHRU_PTIME_MISMATCH);
 		}