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>

drivers/net/ethernet/intel/fm10k/fm10k_pci.c

@@ -1143,14 +1143,16 @@ static irqreturn_t fm10k_msix_mbx_pf(int __always_unused irq, void *data)
 
 void fm10k_mbx_free_irq(struct fm10k_intfc *interface)
 {
-	struct msix_entry *entry = &interface->msix_entries[FM10K_MBX_VECTOR];
 	struct fm10k_hw *hw = &interface->hw;
+	struct msix_entry *entry;
 	int itr_reg;
 
 	/* no mailbox IRQ to free if MSI-X is not enabled */
 	if (!interface->msix_entries)
 		return;
 
+	entry = &interface->msix_entries[FM10K_MBX_VECTOR];
+
 	/* disconnect the mailbox */
 	hw->mbx.ops.disconnect(hw, &hw->mbx);