Silence minor compiler warnings.

Silence minor compiler warnings.

Depending on compiler version and optimization level, we might
get a complaint that lazy_scan_heap's "freespace" is used
uninitialized.

Compilers not aware that ereport(ERROR) doesn't return complained
about bbsink_lz4_new().

Assigning "-1" to a uint64 value has unportable results; fortunately,
the value of xlogreadsegno is unimportant when xlogreadfd is -1.
(It looks to me like there is no need for xlogreadsegno to be static
in the first place, but I didn't venture to change that.)