[OpenMP][libomptarget] Set the frame pointer then test empty slot condition

Summary: NFC - just fixing a bug: the empty slot test was before the re-setting of the Stack pointer. 

Reviewers: ABataev, caomhin, Hahnfeld

Reviewed By: ABataev

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D52122

llvm-svn: 343006