perf buildid: Do not assume that readlink() returns a null terminated string

perf buildid: Do not assume that readlink() returns a null terminated string

Valgrind was complaining:

  $ valgrind ./perf list >/dev/null
  ==11643== Memcheck, a memory error detector
  ==11643== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
  ==11643== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
  ==11643== Command: ./perf list
  ==11643==
  ==11643== Conditional jump or move depends on uninitialised value(s)
  ==11643==    at 0x4C30620: rindex (vg_replace_strmem.c:199)
  ==11643==    by 0x49DAA9: build_id_cache__origname (build-id.c:198)
  ==11643==    by 0x49E1C7: build_id_cache__valid_id (build-id.c:222)
  ==11643==    by 0x49E1C7: build_id_cache__list_all (build-id.c:507)
  ==11643==    by 0x4B9C8F: print_sdt_events (parse-events.c:2067)
  ==11643==    by 0x4BB0B3: print_events (parse-events.c:2313)
  ==11643==    by 0x439501: cmd_list (builtin-list.c:53)
  ==11643==    by 0x497150: run_builtin (perf.c:359)
  ==11643==    by 0x428CE0: handle_internal_command (perf.c:421)
  ==11643==    by 0x428CE0: run_argv (perf.c:467)
  ==11643==    by 0x428CE0: main (perf.c:614)
  [...]

Additionally, a zero length result from readlink() is not very interesting.

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170322130624.21881-3-tommi.t.rantala@nokia.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>