LibIMAP: Make parsing of atom data fallible

We now return an error where `parse_atom()` would have previously
returned an empty StringView. This is consistent with RFC3501, which
says that an atom consists of one or more characters.

This prevents a few cases where parsing an invalid atom could lead to
an infinite loop.

Userland/Libraries/LibIMAP/Parser.h

@@ -41,7 +41,7 @@ private:
     ErrorOr<void> parse_untagged();
     ErrorOr<void> parse_capability_response();
 
-    StringView parse_atom();
+    ErrorOr<StringView> parse_atom();
     ErrorOr<StringView> parse_quoted_string();
     ErrorOr<StringView> parse_literal_string();
     ErrorOr<StringView> parse_string();