[perl #123538] always set chophere and itembytes at the same time

Previously this would crash in FF_MORE because chophere was still
NULL.

pp_ctl.c

@@ -586,6 +586,7 @@ PP(pp_formline)
                         break;
                 }
                 itembytes = s - item;
+                chophere = s;
 		break;
 	    }
 

t/op/write.t

@@ -98,7 +98,7 @@ for my $tref ( @NumTests ){
 my $bas_tests = 21;
 
 # number of tests in section 3
-my $bug_tests = 66 + 3 * 3 * 5 * 2 * 3 + 2 + 66 + 4 + 2 + 3 + 96 + 11 + 2;
+my $bug_tests = 66 + 3 * 3 * 5 * 2 * 3 + 2 + 66 + 4 + 2 + 3 + 96 + 11 + 3;
 
 # number of tests in section 4
 my $hmb_tests = 37;
@@ -1960,6 +1960,18 @@ dd|
 EXPECT
 	      { stderr => 1 }, '#123245 different panic in sv_chop');
 
+fresh_perl_is(<<'EOP', <<'EXPECT',
+format STDOUT =
+# x at the end to make the spaces visible
+@... x
+q/a/
+.
+write;
+EOP
+a    x
+EXPECT
+	      { stderr => 1 }, '#123538 crash in FF_MORE');
+
 #############################
 ## Section 4
 ## Add new tests *above* here