debuginfo: Don't crash when encountering global variable with unknown source span.
auto merge of #15719 : michaelwoerister/rust/global_var_null_span_fix, r=alexcrichton

This should fix issue #15541. It would be good to have an test case for this would also be nice but I haven't had the time to write one. The change is very small though and it doesn't break anything in the existing test suite, so I guess we can add it without test for now.