[MLIR][NFC] Fix a memset in MemRefUtils.h

found by PVS-Studio - https://pvs-studio.com/en/blog/posts/cpp/1003/, N10.
memset function expects to take int as the second actual argument,
but receives a pointer. Here, the first and the second argument of
the function are mixed up.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D142310