ps: fix incorrect trimming

[Job Snijders]

Christian Weisgerber reported the new 'f' feature in ps(1) does not
apply line length trimming correctly.

The problem can be observed when comparing '$ COLUMNS=79 ps l' and
'$ COLUMNS=79 ps lf'

OK?

Index: print.c
===================================================================
RCS file: /cvs/src/bin/ps/print.c,v
retrieving revision 1.83
diff -u -p -r1.83 print.c
--- print.c     1 Sep 2022 21:15:54 -0000       1.83
+++ print.c     19 Sep 2022 19:44:58 -0000
@@ -140,7 +140,7 @@ command(const struct pinfo *pi, VARENT *
 
        if (needcomm) {
                if (pi->prefix)
-                       mbswprint(pi->prefix, left, 0);
+                       left -= mbswprint(pi->prefix, left, 0);
                if (!commandonly) {
                        char **argv = NULL;