evutil_parse_sockaddr_port(): fix buffer overflow

@asn-the-goblin-slayer:
  "Length between '[' and ']' is cast to signed 32 bit integer on line 1815. Is
   the length is more than 2<<31 (INT_MAX), len will hold a negative value.
   Consequently, it will pass the check at line 1816. Segfault happens at line
   1819.

   Generate a resolv.conf with generate-resolv.conf, then compile and run
   poc.c. See entry-functions.txt for functions in tor that might be
   vulnerable.

   Please credit 'Guido Vranken' for this discovery through the Tor bug bounty
   program."

Reproducer for gdb (https://gist.github.com/azat/be2b0d5e9417ba0dfe2c):
  start
  p (1ULL<<31)+1ULL
  # $1 = 2147483649
  p malloc(sizeof(struct sockaddr))
  # $2 = (void *) 0x646010
  p malloc(sizeof(int))
  # $3 = (void *) 0x646030
  p malloc($1)
  # $4 = (void *) 0x7fff76a2a010
  p memset($4, 1, $1)
  # $5 = 1990369296
  p (char *)$4
  # $6 = 0x7fff76a2a010 '\001' <repeats 200 times>...
  set $6[0]='['
  set $6[$1]=']'
  p evutil_parse_sockaddr_port($4, $2, $3)
  # $7 = -1

Before:
  $ gdb bin/http-connect < gdb
  (gdb) $1 = 2147483649
  (gdb) (gdb) $2 = (void *) 0x646010
  (gdb) (gdb) $3 = (void *) 0x646030
  (gdb) (gdb) $4 = (void *) 0x7fff76a2a010
  (gdb) (gdb) $5 = 1990369296
  (gdb) (gdb) $6 = 0x7fff76a2a010 '\001' <repeats 200 times>...
  (gdb) (gdb) (gdb) (gdb)
  Program received signal SIGSEGV, Segmentation fault.
  __memcpy_sse2_unaligned () at memcpy-sse2-unaligned.S:36

After:
  $ gdb bin/http-connect < gdb
  (gdb) $1 = 2147483649
  (gdb) (gdb) $2 = (void *) 0x646010
  (gdb) (gdb) $3 = (void *) 0x646030
  (gdb) (gdb) $4 = (void *) 0x7fff76a2a010
  (gdb) (gdb) $5 = 1990369296
  (gdb) (gdb) $6 = 0x7fff76a2a010 '\001' <repeats 200 times>...
  (gdb) (gdb) (gdb) (gdb) $7 = -1
  (gdb) (gdb) quit

Fixes: #318
evutil_memclear_(void *mem, size_t len)
{
	evutil_memset_volatile_(mem, 0, len);
}

evutil_memclear_(void *mem, size_t len)
{
	evutil_memset_volatile_(mem, 0, len);
}
