perf/x86/utils: Fix uninitialized var in get_branch_type()

offset is passed as a pointer and on certain call path is not set by the
function. If the caller does not re-initialize offset between calls, value
could be inherited between calls. Prevent this by initializing offset on each
call.

This impacts the code in amd_pmu_lbr_filter() which does:

   for(i=0; ...) {
       ret = get_branch_type_fused(..., &offset);
       if (offset)
          lbr_entries[i].from += offset;
   }

Fixes: df3e9612 ("perf/x86: Make branch classifier fusion-aware")
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Link: https://lore.kernel.org/r/20220928184043.408364-2-eranian@google.com