fil_write_flushed_lsn(): Ensure that the return value is initialized

Previously, the function could theoretically return an uninitialized
value if the system tablespace contained no data files. It should be
impossible for InnoDB to start up in such scenario.

storage/innobase/fil/fil0fil.cc

@@ -2074,7 +2074,7 @@ fil_write_flushed_lsn(
 {
 	byte*	buf1;
 	byte*	buf;
-	dberr_t	err;
+	dberr_t	err = DB_TABLESPACE_NOT_FOUND;
 
 	buf1 = static_cast<byte*>(ut_malloc(2 * UNIV_PAGE_SIZE));
 	buf = static_cast<byte*>(ut_align(buf1, UNIV_PAGE_SIZE));

storage/xtradb/fil/fil0fil.cc

@@ -2125,7 +2125,7 @@ fil_write_flushed_lsn(
 {
 	byte*	buf1;
 	byte*	buf;
-	dberr_t	err;
+	dberr_t	err = DB_TABLESPACE_NOT_FOUND;
 
 	buf1 = static_cast<byte*>(ut_malloc(2 * UNIV_PAGE_SIZE));
 	buf = static_cast<byte*>(ut_align(buf1, UNIV_PAGE_SIZE));