core.internal.backtrace.dwarf: Fix segfault regression in case executable cannot be opened

[kinke]

Introduced by #3322. image.baseAddress() must not be called if !image.isValid(), otherwise a segfault occurs if the executable file/image could not be opened (e.g., because of the process changing the initial working dir and https://issues.dlang.org/show_bug.cgi?id=21656).

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Testing this PR locally

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[PetarKirov]

Thanks for the fix, LGTM.

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Some extra detail for other reviewers:

Given that the baseAddress parameter of processCallstack is only used when debugLineSectionData parameter is not null, it may seem like passing 0 for it here is of no consequence. However Image.baseAddress is actually a member function property which triggers the bug that this PR fixes. I must have overlooked this detail here: https://github.com/dlang/druntime/pull/3322/files#diff-7c0ebe7e385f5fadd65ebb53ce4ceaad90d419af813405a5efb828fb99deea79R181-R184