rust: Replace two `unwrap()`s in FFI code with `unwrap_or()`s.

src/rust/protover/ffi.rs

@@ -232,7 +232,7 @@ pub extern "C" fn protover_compute_for_old_tor(version: *const c_char) -> *const
     // we can see that the bytes we're passing into it 1) are valid UTF-8,
     // 2) have no intermediate NUL bytes, and 3) are terminated with a NUL
     // byte.
-    supported = CStr::from_bytes_with_nul(elder_protocols).unwrap();
+    supported = CStr::from_bytes_with_nul(elder_protocols).unwrap_or(empty);
 
     supported.as_ptr()
 }

src/rust/protover/protover.rs

@@ -110,7 +110,7 @@ pub fn get_supported_protocols() -> &'static str {
     // The `unwrap` is safe becauase we SUPPORTED_PROTOCOLS is under
     // our control.
     str::from_utf8(&SUPPORTED_PROTOCOLS[..SUPPORTED_PROTOCOLS.len() - 1])
-        .unwrap()
+        .unwrap_or("")
 }
 
 pub struct SupportedProtocols(HashMap<Proto, Versions>);