Tweak spawn table creation to not overflow.

src/core.rs

@@ -522,10 +522,12 @@ impl<T> RandomTable<T>
 where
     T: Clone,
 {
-    pub fn add(&mut self, value: T, weight: u32) -> &mut Self {
+    pub fn add(&mut self, value: T, weight: i32) -> &mut Self {
+        if weight > 0 {
             for _ in 0..weight {
                 self.0.push(value.clone());
             }
+        }
         self
     }
 }