test: fix uninitialized memory warning, use calloc
test: fix uninitialized memory warning, use calloc

Fixes the following valgrind warning:

  ==29019== Syscall param writev(vector[...]) points to uninitialised byte(s)
  ==29019==    at 0x584270B: writev (writev.c:51)
  ==29019==    by 0x449BB2: uv__write (stream.c:733)
  ==29019==    by 0x44AE91: uv_write2 (stream.c:1159)
  ==29019==    by 0x44AF25: uv_write (stream.c:1180)
  ==29019==    by 0x42CCAA: connect_cb (test-tcp-writealot.c:129)
  ==29019==    by 0x44AC05: uv__stream_connect (stream.c:1097)
  ==29019==    by 0x44AA25: uv__stream_io (stream.c:1050)
  ==29019==    by 0x437430: uv__io_rw (core.c:539)
  ==29019==    by 0x43C3D9: ev_invoke_pending (ev.c:2145)
  ==29019==    by 0x436EC5: uv__poll (core.c:260)
  ==29019==    by 0x436F0F: uv__run (core.c:269)
  ==29019==    by 0x436F6E: uv_run (core.c:277)
  ==29019==  Address 0x5f15040 is 0 bytes inside a block of size 94,371,840 alloc'd
  ==29019==    at 0x4C2C5EF: malloc (vg_replace_malloc.c:270)
  ==29019==    by 0x42CDED: run_test_tcp_writealot (test-tcp-writealot.c:148)
  ==29019==    by 0x406551: run_test_part (runner.c:302)
  ==29019==    by 0x405384: main (run-tests.c:57)

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso