proper error message on missing X display

assert is inappropriate here because it is optimized out when compiling
with NDEBUG

1 files changed, 5 insertions, 2 deletions

diff --git a/src/xlib.c b/src/xlib.c
index 2912321..ebc3a4c 100644
--- a/src/xlib.c
+++ b/src/xlib.c
@@ -172,8 +172,11 @@ static int xquit(Display *d){
 }
 
 void backend_init(){
-	display = XOpenDisplay (NULL);
-	assert(display);
+	display = XOpenDisplay(NULL);
+	if(!display){
+		fprintf(stderr, "Can't open X display.\n");
+		exit(EXIT_FAILURE);
+	}
 	xfd = ConnectionNumber(display);
 	screen = DefaultScreen(display);