Set NODELETE flag after checking for NULL pointer

The commit b632bdd3 moved the setting of the DF_1_NODELETE flag earlier
in the dl_open_worker function. However when calling dlopen with both
RTLD_NODELETE and RTLD_NOLOAD, the pointer returned by _dl_map_object is
NULL. This condition is checked just after setting the flag, while it
should be done before. Fix that.

Changelog:
	[BZ #19810]
	* elf/dl-open.c (dl_open_worker): Set DF_1_NODELETE flag later.
	* elf/tst-noload.c: New test case.
	* elf/Makefile (tests): Add tst-noload.