StandaloneMmPkg/Core: fix bogus FV pointer in DEBUG string

StandaloneMmPkg/Core: fix bogus FV pointer in DEBUG string

FvIsBeingProcessed () emits a DEBUG print with the intent to print
the memory address of the FV that is being processed, but instead,
it prints the contents of an uninitialized stack variable.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>