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/replication/basebackup_lz4.c

@@ -66,6 +66,7 @@ bbsink_lz4_new(bbsink *next, int compresslevel)
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 			 errmsg("lz4 compression is not supported by this build")));
+	return NULL;				/* keep compiler quiet */
 #else
 	bbsink_lz4 *sink;