cgtop: fix assert when not on tty

systemd-cgtop --dept=1 -b -n 10 -d 0.1 | cat

Assertion 'new_length >= 3' failed at src/shared/util.c:3 \
595, function ellipsize_mem(). Aborting.
Aborted (core dumped)

(David: add comment)

(cherry picked from commit 510c4a0f1e)

src/cgtop/cgtop.c

@@ -447,7 +447,7 @@ static int display(Hashmap *a) {
         Group *g;
         Group **array;
         signed path_columns;
-        unsigned rows, n = 0, j, maxtcpu = 0, maxtpath = 0;
+        unsigned rows, n = 0, j, maxtcpu = 0, maxtpath = 3; /* 3 for ellipsize() to work properly */
         char buffer[MAX3(21, FORMAT_BYTES_MAX, FORMAT_TIMESPAN_MAX)];
 
         assert(a);