Correct the return value check for ast_safe_system.

The logic here was reversed as ast_safe_system returns -1 on error and not on
success.  Fix suggested by reporter.

(closes issue #15667)
Reported by: loic


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@212574 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/logger.c

@@ -539,7 +539,7 @@ static int rotate_file(const char *filename)
 		char buf[512];
 		pbx_builtin_setvar_helper(c, "filename", filename);
 		pbx_substitute_variables_helper(c, exec_after_rotate, buf, sizeof(buf));
-		if (ast_safe_system(buf) != -1) {
+		if (ast_safe_system(buf) == -1) {
 			ast_log(LOG_WARNING, "error executing '%s'\n", buf);
 		}
 		c = ast_channel_release(c);