BUILD: h1: silence an initiialized warning with gcc-4.7 and -Os

BUILD: h1: silence an initiialized warning with gcc-4.7 and -Os

Building h1.c with gcc-4.7 -Os produces the following warning:

  src/h1.c: In function 'h1_headers_to_hdr_list':
  src/h1.c:1101:36: warning: 'ptr' may be used uninitialized in this function [-Wmaybe-uninitialized]

In fact ptr may be taken from sl.rq.u.ptr which is only initialized after
passing through the relevant states, but gcc doesn't know which states
are visited. Adding an ALREADY_CHECKED() statement there is sufficient to
shut it up and doesn't affect the emitted code.

This may be backported to stable versions to make sure that builds on older
distros and systems is clean.