analysis: Avoid overflow when determining whether to emit a parameter

[kawadakk]

Fixes a debug build of GIR panicking when processing non-static async methods.

  thread 'main' panicked at 'attempt to subtract with overflow', ...\gtk-rs-gir\src\analysis\function_parameters.rs:243:46
  stack backtrace:
     0: rust_begin_unwind
               at /rustc/4087deaccd8bceb458c9610d29744d5f3504c5c0/library\std\src/panicking.rs:577:5
     1: core::panicking::panic_fmt
               at /rustc/4087deaccd8bceb458c9610d29744d5f3504c5c0/library\core\src/panicking.rs:67:14
     2: core::panicking::panic
               at /rustc/4087deaccd8bceb458c9610d29744d5f3504c5c0/library\core\src/panicking.rs:117:5
     3: libgir::analysis::function_parameters::analyze
     4: libgir::analysis::functions::analyze_function
     5: libgir::analysis::functions::analyze
     6: libgir::analysis::object::interface
     7: libgir::analysis::analyze
     8: libgir::analysis::run
     9: gir::main
               at ...\gtk-rs-gir\src\main.rs:192:9