This fixes a "subtract with overflow" error that could happen in debug mode when viewing the project diagnostics. From git bisecting, I think that this behavior was introduced by https://github.com/zed-industries/zed/pull/21942. It seems like it's possible in some cases for the excerpt-expansion heuristic to cause the excerpt's `context` range to start *after* the excerpt's `primary` range. We should probably revisit that heuristic at some point, but it also seems reasonable to handle that situation at this layer, rather than overflowing. Release Notes: - N/A