csplit: don't panic on missing suppressed file

Avoid a panic that would occur when attempting to remove a file that
didn't exist. This would happen when scanning for a regular expression
match, advancing by a positive offset, and suppressing empty files. For
example, before this commit,

    echo a | csplit -z - %a%1

would cause a panic. After this commit, the process terminates as
expected: without error, without output, and without any files written.

Fixes #7251.

tests/by-util/test_csplit.rs

@@ -335,6 +335,16 @@ fn test_skip_to_match_offset() {
     }
 }
 
+#[test]
+fn test_skip_to_match_offset_suppress_empty() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    ucmd.args(&["-z", "-", "%a%1"])
+        .pipe_in("a\n")
+        .succeeds()
+        .no_output();
+    assert!(!at.file_exists("xx00"));
+}
+
 #[test]
 fn test_skip_to_match_negative_offset() {
     let (at, mut ucmd) = at_and_ucmd!();