fm10k: prevent null pointer dereference of msix_entries table

According to the C standard dereferencing a variable before it is
checked invokes undefined behavior, and thus compilers are free to
assume the check for NULL isn't necessary. Prevent this by re-ordering
the NULL check of msix_entries in fm10k_free_mbx_irq.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <Krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>