Remove an allocation in min_stack

Signed-off-by: Alex Saveau <[email protected]>

library/std/src/sys_common/thread.rs

@@ -8,7 +8,7 @@ pub fn min_stack() -> usize {
         0 => {}
         n => return n - 1,
     }
-    let amt = env::var("RUST_MIN_STACK").ok().and_then(|s| s.parse().ok());
+    let amt = env::var_os("RUST_MIN_STACK").and_then(|s| s.to_str().and_then(|s| s.parse().ok()));
     let amt = amt.unwrap_or(imp::DEFAULT_MIN_STACK_SIZE);
 
     // 0 is our sentinel value, so ensure that we'll never see 0 after