perf annotate: Fix crashes on empty annotate windows

perf annotate: Fix crashes on empty annotate windows

Annotate can open with an empty window if the disassembly tool fails.
After the linked change, the TUI started assuming there was a current
annotation line and could assert or segfault in the seek, refresh, and
source-toggle paths.

Handle empty annotate windows explicitly: set the asm entry count before
resetting the browser, return early when refreshing an empty list, and
ignore source line toggle when there is no current annotation line.

Fixes the following when opening an annotation:

  perf: ui/browser.c:125: ui_browser__list_head_seek: Assertion `pos != NULL' failed.
  Aborted

Fixes: e201757f7a0a901e ("perf annotate: Fix source code annotate with objdump")
Assisted-by: GitHub Copilot:GPT-5.4
Signed-off-by: James Clark <james.clark@linaro.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>