[patch] proposed fix for libstdc++/59829

[Jonathan Wakely <jwakely at redhat dot com>]

This is the best I've come up with to avoid dereferencing an invalid
pointer when calling vector::data() on an empty vector.

For C++03 we reurn the vector's pointer type, so can just return the
internal pointer, but for C++11 we need to convert that to a raw
pointer, which we do by dereferencing, so we must check if it's valid
first.

Any better suggestions before I commit this?
(Tests pass, although I need to adjust some dg-error line numbers
before committing it.)

Attachment: patch.txt
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