gdb/dap: fix decode_source

The documentation for the Source interface says

   * The path of the source to be shown in the UI.
   * It is only used to locate and load the content of the source if no
   * `sourceReference` is specified (or its value is 0).

but the code used `path` first. I fixed it to use `sourceReference` first.

Approved-By: Tom Tromey <tom@tromey.com>
1 file changed