appender: fix max_files integer underflow when set to zero (#3348)

## Motivation

When `max_files` set to zero,
[rolling.rs:695](https://github.com/tokio-rs/tracing/blob/c01d4fd9def2fb061669a310598095c789ca0a32/tracing-appender/src/rolling.rs#L695)
will cause integer underflow and panicking in debug build. In my
opinion the API should prevent this from happening as "zero" is a
valid `usize` value to pass in.

## Solution

Currently, in release builds, if `max_files` is set to zero, it
behaves just like `u64::MAX` on `x86_64` platforms. Therefore, I
decided to simply make `zero == None`, which should align with the
existing behavior but without the panicking. I believe this can be
considered as "non-breaking".

Author: unlimitedsola

tracing-appender/src/rolling/builder.rs

@@ -195,7 +195,7 @@ impl Builder {
     /// the maximum, so if you need to retain `m` log files, specify a max of
     /// `m + 1`.
     ///
-    /// If no value is supplied, the `RollingAppender` will not remove any files.
+    /// If `0` is supplied, the `RollingAppender` will not remove any files.
     ///
     /// Files are considered candidates for deletion based on the following
     /// criteria:
@@ -232,7 +232,8 @@ impl Builder {
     #[must_use]
     pub fn max_log_files(self, n: usize) -> Self {
         Self {
-            max_files: Some(n),
+            // Setting `n` to 0 will disable the max files (effectively make it infinite).
+            max_files: Some(n).filter(|&n| n > 0),
             ..self
         }
     }