fix(parser): Don't panic on --=

This broke when we introduced clap_lex and then did a refactor on top.
We put in guards to say that escapes shouldn't happen but missed `--=`
which isn't quite an escape.

Not fully set on what error should be returned in this case (we are
returning roughly what we used to) but at least
we aren't panicing.

Fixes #3858

src/parser/parser.rs

@@ -738,8 +738,10 @@ impl<'help, 'cmd> Parser<'help, 'cmd> {
             }
         };
         if long_arg.is_empty() {
-            debug_assert!(long_value.is_none(), "{:?}", long_value);
-            return Ok(ParseResult::NoArg);
+            debug_assert!(
+                long_value.is_some(),
+                "`--` should be filtered out before this point"
+            );
         }
 
         let arg = if let Some(arg) = self.cmd.get_keymap().get(long_arg) {