src: Finish assert() clean-up

From cleanup commit 0cecde304:
assert()s can be compiled away by #defining NDEBUG. Some build systems
do this. Using wl_abort gives a human readable error message and it
isn't compiled away.

That commit missed one final assert, presumably due to missing it with
grep because of a coding style issue. Fix that up, and remove inclusion
of <assert.h> as appropriate.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

src/connection.c

@@ -26,7 +26,6 @@
 
 #define _GNU_SOURCE
 
-#include <assert.h>
 #include <math.h>
 #include <stdlib.h>
 #include <stdint.h>