key_buffer_size.

The cause of corruption was number overflow when multiplying 
two ulong values, number of used keycache blocks with size
of a single block. The result of multiplication exceeded ulong 
range (4G) and this lead to incorrectly calculated  buffer offset
in the key cache.

The fix is to use size_t for multiplication result.

This patch also fixes pointless cast in safemalloc 
(size of allocated block to uint), that creates lot of false
alarm warnings when using big keycache (> 4GB) in debug mode.