Update to handle dsym files that end with a header.
Update to handle dsym files that end with a header.

dsym files generated by Xcode for swift (Xcode 7.3) end with a header, and the
code did not handle that case.

BUG=https://bugs.chromium.org/p/google-breakpad/issues/detail?id=689
R=ivanpe@chromium.org

Review URL: https://codereview.chromium.org/1971793002 .

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