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.
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