[OpenMP][Offloading] Fix test case issues in bug49334.cpp

`bug49334.cpp` has one issue that causes flaky result reported in #53730.
The root cause is `BlockedC` is never initialized but in `BlockMatMul_TargetNowait`
it is directly read and written (via `+=`). Fixes #53730.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D119988