Bootstrap-O3 complains about uninitialized uses of this variable in
gengtype-state.c.  The variable can never actually be used before
initialization, but GCC is not smart enough to realize that.
read_state_structures, just before the function at hand, has a similar
loop and it zero-initializes both head and previous, so I adjusted the
affected function to follow the same pattern and avoid the bogus
warning.  Regstrapped on x86_64-linux-gnu and i686-pc-linux-gnu.  Ok to
install?