Reject zero or negative SMOL_THREADS value (#382)

SMOL_THREADS=0 creates a range 1..=0 which yields no worker
threads, causing tasks spawned with smol::spawn to hang forever.
Non-numeric values already fall back to 1 because parse fails,
but 0 parsed successfully and produced an empty range.

Add .filter(|&n| n > 0) so that a zero value falls back to
the default of 1, matching the behavior for invalid values.

Closes #381

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

Author: cschanhniem

src/spawn.rs

@@ -39,7 +39,8 @@ pub fn spawn<T: Send + 'static>(future: impl Future<Output = T> + Send + 'static
                 // Parse SMOL_THREADS or default to 1.
                 std::env::var("SMOL_THREADS")
                     .ok()
-                    .and_then(|s| s.parse().ok())
+                    .and_then(|s| s.parse::<usize>().ok())
+                    .filter(|&n| n > 0)
                     .unwrap_or(1)
             };