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.)

src/backend/access/heap/vacuumlazy.c

@@ -1112,7 +1112,7 @@ lazy_scan_heap(LVRelState *vacrel, int nworkers)
 			if (lazy_scan_noprune(vacrel, buf, blkno, page, &hastup,
 								  &recordfreespace))
 			{
-				Size		freespace;
+				Size		freespace = 0;
 
 				/*
 				 * Processed page successfully (without cleanup lock) -- just