MINOR: ncbuf: missing malloc checks in standalone code

Some malloc resulsts were not checked in standalone ncbuf code.

As this is debug/test code, we don't need to explicitly handle memory
errors, we just add some BUG_ON() to ensure that memory is properly
allocated and prevent unexpected results.

This partially fixes issue GH #2130.

No backport needed.

src/ncbuf.c

@@ -810,9 +810,11 @@ static int ncbuf_test(ncb_sz_t head, int reset, int print_delay)
 	enum ncb_ret ret;
 
 	data0 = malloc(bufsize);
+	BUG_ON(!data0);
 	memset(data0, 0xff, bufsize);
 
 	bufarea = malloc(bufsize);
+	BUG_ON(!bufarea);
 
 	fprintf(stderr, "running unit tests\n");