subscriber: fix clippy lints (#636)

## Motivation

Looks like Clippy is once again complaining about the
`match_wild_err_arm` lint, presumably as a result of the Rust 1.42
release. This lint triggers on the `try_lock!` macro that
`tracing-subscriber` uses to avoid double panics when a mutex is
poisoned. In this case, the lint is something of a false positive here,
since we _do_ actually have two different `Err(...)` arms; the
differentiation between the two arms is not in the match pattern but in
a guard. See rust-lang/rust-clippy#3688 for
details on the lint.

## Solution

I've refactored the code in question to use `if`/`else`, avoiding the
lint.

Signed-off-by: Eliza Weisman <[email protected]>

tracing-subscriber/src/lib.rs

@@ -83,10 +83,12 @@ macro_rules! try_lock {
         try_lock!($lock, else return)
     };
     ($lock:expr, else $els:expr) => {
-        match $lock {
-            Ok(l) => l,
-            Err(_err) if std::thread::panicking() => $els,
-            Err(_err) => panic!("lock poisoned"),
+        if let Ok(l) = $lock {
+            l
+        } else if std::thread::panicking() {
+            $els
+        } else {
+            panic!("lock poisoned")
         }
     };
 }