Fix minor problems found by static checking.

Fix minor problems found by static checking.

* bidi.c (bidi_cache_size): Now EMACS_INT, not size_t.
(elsz): Now a signed constant, not a size_t var.  We prefer signed
types to unsigned, to avoid integer comparison confusion.  Without
this change, GCC 4.6.1 with -Wunsafe-loop-optimizations complains
"cannot optimize loop, the loop counter may overflow", a symptom
of the confusion.