Check the return value of lo_export.

Patch by Ondřej Surý. Fixes bug #68697 (lo_export return -1 on failure).
Merge branch 'PHP-5.5' into PHP-5.6

* PHP-5.5:
  Check the return value of lo_export.
Merge branch 'PHP-5.6'

* PHP-5.6:
  Check the return value of lo_export.