perf annotate: Fix 's' on source line when disasm is empty

If the disasm is empty, 's' should fail. Instead it seemingly works,
hiding the empty lines and causing an assertion error on the next time
annotate is called (from within perf report).

The problem is caused by a buffer overflow, caused by a wrong exit
condition in annotate_browser__find_next_asm_line, which checks
browser->b.top instead of browser->b.entries.

This patch fixes the issue, making annotate_browser__toggle_source
fail if the disasm is empty (nothing happens to the user).

Fixes: 6de249d66d2e7881 ("perf annotate: Allow 's' on source code lines")
Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin Liška <mliska@suse.cz>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210705161524.72953-1-rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>