pgsql: Don't crash when pg_xlog is empty and pg_basebackup -x is used

Don't crash when pg_xlog is empty and pg_basebackup -x is used

The backup will not work (without a logarchive, and that's the whole
point of -x) in this case, this patch just changes it to throw an
error instead of crashing when this happens.

Noticed and diagnosed by TAKATSUKA Haruka

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/3979ff1fc6e9dd7847ff9bb24beb4d03ebe02cef

Modified Files
--------------
src/backend/replication/basebackup.c |    8 ++++++++
1 file changed, 8 insertions(+)