[gdb/rust] Fix literal truncation

Make sure we error out on overflow instead of truncating in all cases.

I've used as overflow string: "Integer literal is too large", based
on what I found at
<rust-lang/rust>/src/test/ui/parser/int-literal-too-large-span.rs
but perhaps someone has a better idea.

Tested on x86_64-linux, with a build with --enable-targets=all.