Log Message:
exim: fix crash on startup if log_buffer is allocated right after taint pool

The check whether a block of memory is tainted erroneously returns true
if the block in question starts the very next byte after a block in the
tainted pool. Depending on the memory allocator, this can cause problems.
For example, on NetBSD/amd64 9.0, this seems to allocate the first tainted
block immediately before log_buffer. This leads to a recursive error in
log_write the first time anything is written to the log, leading to a
segmentation fault when the stack fills up.