perf utils: Null terminate buf in read_ftrace_printk()

perf utils: Null terminate buf in read_ftrace_printk()

Ensure that the string that we read from the data file is null terminated.

Valgrind was complaining:

  ==31357== Invalid read of size 1
  ==31357==    at 0x4EC8C1: __strtok_r_1c (string2.h:200)
  ==31357==    by 0x4EC8C1: parse_ftrace_printk (trace-event-parse.c:161)
  ==31357==    by 0x4F82A8: read_ftrace_printk (trace-event-read.c:204)
  ==31357==    by 0x4F82A8: trace_report (trace-event-read.c:468)
  ==31357==    by 0x4CD552: process_tracing_data (header.c:1576)
  ==31357==    by 0x4D3397: perf_file_section__process (header.c:2705)
  ==31357==    by 0x4D3397: perf_header__process_sections (header.c:2488)
  ==31357==    by 0x4D3397: perf_session__read_header (header.c:2925)
  ==31357==    by 0x4E71E2: perf_session__open (session.c:32)
  ==31357==    by 0x4E71E2: perf_session__new (session.c:139)
  ==31357==    by 0x429F5D: cmd_annotate (builtin-annotate.c:472)
  ==31357==    by 0x497150: run_builtin (perf.c:359)
  ==31357==    by 0x428CE0: handle_internal_command (perf.c:421)
  ==31357==    by 0x428CE0: run_argv (perf.c:467)
  ==31357==    by 0x428CE0: main (perf.c:614)
  ==31357==  Address 0x8ac0efb is 0 bytes after a block of size 1,963 alloc'd
  ==31357==    at 0x4C2DB9D: malloc (vg_replace_malloc.c:299)
  ==31357==    by 0x4F827B: read_ftrace_printk (trace-event-read.c:195)
  ==31357==    by 0x4F827B: trace_report (trace-event-read.c:468)
  ==31357==    by 0x4CD552: process_tracing_data (header.c:1576)
  ==31357==    by 0x4D3397: perf_file_section__process (header.c:2705)
  ==31357==    by 0x4D3397: perf_header__process_sections (header.c:2488)
  ==31357==    by 0x4D3397: perf_session__read_header (header.c:2925)
  ==31357==    by 0x4E71E2: perf_session__open (session.c:32)
  ==31357==    by 0x4E71E2: perf_session__new (session.c:139)
  ==31357==    by 0x429F5D: cmd_annotate (builtin-annotate.c:472)
  ==31357==    by 0x497150: run_builtin (perf.c:359)
  ==31357==    by 0x428CE0: handle_internal_command (perf.c:421)
  ==31357==    by 0x428CE0: run_argv (perf.c:467)
  ==31357==    by 0x428CE0: main (perf.c:614)

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/20170322130624.21881-6-tommi.t.rantala@nokia.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>