Fix panic when using negative int for bytes()

Fixes #1401

Cargo.lock

@@ -1221,6 +1221,7 @@ dependencies = [
  "cfg-if 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "futures 0.1.29 (registry+https://github.com/rust-lang/crates.io-index)",
  "js-sys 0.3.27 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-bigint 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustpython-compiler 0.1.0",
  "rustpython-parser 0.1.0",

tests/snippets/basic_types.py

@@ -1,3 +1,6 @@
+from testutils import assert_raises
+
+
 # Spec: https://docs.python.org/2/library/types.html
 print(None)
 # TypeType
@@ -36,6 +39,9 @@ try:
 except TypeError:
     pass
 
+with assert_raises(ValueError):
+    bytes(-1)
+
 a = bytearray([1, 2, 3])
 # assert a[1] == 2
 

vm/src/obj/objbyteinner.rs

@@ -117,7 +117,9 @@ impl ByteInnerNewOptions {
             let value = if let OptionalArg::Present(ival) = self.val_option {
                 match_class!(match ival.clone() {
                     i @ PyInt => {
-                        let size = objint::get_value(&i.into_object()).to_usize().unwrap();
+                        let size = objint::get_value(&i.into_object())
+                            .to_usize()
+                            .ok_or_else(|| vm.new_value_error("negative count".to_string()))?;
                         Ok(vec![0; size])
                     }
                     _l @ PyString => {