expr.c (expr_expand_equ): Work around what I believe to be a GCC aliasing

expr.c (expr_expand_equ): Work around what I believe to be a GCC aliasing
bug (probably http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28778) by not
accepting NULL for eh, and giving it the static eh at the sole caller of
this function (yasm_expr__level_tree).  While I'm here, clean up the logic
a bit to avoid relying on ee.e NULL value.  This fix prevents crashes in
GCC 4.1.1 with -fstrict-aliasing (enabled by -O2).

Noticed by: Danny Reeves <danny_reeves@hotmail.com>

svn path=/trunk/yasm/; revision=1665