made gdb pretty-printing script more robust when printing uninitialized vec.

I based this solution on my reading of:

https://rethinkdb.com/blog/make-debugging-easier-with-custom-pretty-printers#what-is-still-to-be-done

That post claims that there is no clean way to check for garbage pointers, and
so this PR adopts the same solution of tentatively attempting to convert a
dererence to a string, which throws a clean exception on garbage that we can
catch and recover from.

I only made the change to vec and not the other pretty printers because I wanted
to focus my effort on the simplest thing that would resolve issue #64343. In
particular, I *considered* generalizing this fix to work on the other datatypes
in the pretty-printing support library, but I don't want to invest effort in
that until after we resolve our overall debugging support strategy; see also
issues #60826 and #65564.

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