fix: Stop lonely hyphens from causing panic

The method `starts_with` as implemented for the `OsStrExt2` trait on
`OsStr` assumed that the needle given is shorter than the haystack. When
this is not the case, the method panics due to an attempted
out-of-bounds access on the byte representation of `self`. Problematic
if, say, an end-user gives us `"-"` and the library tries to see if that
starts with `"--"`.

Fortunately, slices already implement a `starts_with` method, and we can
delegate to it.

This *does* create a semantics change: if both `self` and the needle
have length 0, this implementation will return `true`, but the old
implementation would return `false`. Based on the test suite still
passing, acknowledging the vacuous truth doesn't seem to cause any
problems.

Fixes #410

src/osstringext.rs

@@ -36,12 +36,7 @@ impl OsStrExt3 for OsStr {
 
 impl OsStrExt2 for OsStr {
     fn starts_with(&self, s: &[u8]) -> bool {
-        let sab = self.as_bytes();
-        if sab.is_empty() { return false; }
-        for (i, b) in s.iter().enumerate() {
-            if *b != sab[i] { return false; }
-        }
-        true
+        self.as_bytes().starts_with(s)
     }
 
     fn is_empty(&self) -> bool {

tests/positionals.rs

@@ -132,3 +132,10 @@ fn create_positional() {
                             .help("testing testing"))
                 .get_matches();
 }
+
+#[test]
+fn positional_hyphen_does_not_panic() {
+    let _ = App::new("test")
+        .arg(Arg::with_name("dummy"))
+        .get_matches_from(vec!["test", "-"]);
+}