per realloc above the length of the buffer is alloc_size, not BUFSIZ.
Adjust length per size as done for buf start.

Addresses some valgrind complaints:

==1870== Syscall param read(buf) points to unaddressable byte(s)
==1870==    at 0x4E3F610: __read_nocancel (in /lib64/libpthread-2.14.90.so)
==1870==    by 0x44AEE1: event_format__new (unistd.h:45)
==1870==    by 0x44B025: perf_evsel__newtp (evsel.c:158)
==1870==    by 0x451919: add_tracepoint_event (parse-events.c:395)
==1870==    by 0x479815: parse_events_parse (parse-events.y:292)
==1870==    by 0x45463A: parse_events_option (parse-events.c:861)
==1870==    by 0x44FEE4: get_value (parse-options.c:113)
==1870==    by 0x450767: parse_options_step (parse-options.c:192)
==1870==    by 0x450C40: parse_options (parse-options.c:422)
==1870==    by 0x42735F: cmd_record (builtin-record.c:918)
==1870==    by 0x419D72: run_builtin (perf.c:319)
==1870==    by 0x4195F2: main (perf.c:376)
==1870==  Address 0xcffebf0 is 0 bytes after a block of size 8,192 alloc'd
==1870==    at 0x4C2A62F: malloc (vg_replace_malloc.c:270)
==1870==    by 0x4C2A7A3: realloc (vg_replace_malloc.c:662)
==1870==    by 0x44AF07: event_format__new (evsel.c:121)
==1870==    by 0x44B025: perf_evsel__newtp (evsel.c:158)
==1870==    by 0x451919: add_tracepoint_event (parse-events.c:395)
==1870==    by 0x479815: parse_events_parse (parse-events.y:292)
==1870==    by 0x45463A: parse_events_option (parse-events.c:861)
==1870==    by 0x44FEE4: get_value (parse-options.c:113)
==1870==    by 0x450767: parse_options_step (parse-options.c:192)
==1870==    by 0x450C40: parse_options (parse-options.c:422)
==1870==    by 0x42735F: cmd_record (builtin-record.c:918)
==1870==    by 0x419D72: run_builtin (perf.c:319)
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1372793245-4136-2-git-send-email-dsahern@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>