Get the ACPI softc before we potentially dereference it.

sys/dev/acpica/Osd/OsdInterrupt.c

@@ -57,6 +57,11 @@ AcpiOsInstallInterruptHandler(UINT32 InterruptNumber, OSD_HANDLER ServiceRoutine
 
     FUNCTION_TRACE(__func__);
 
+    if ((sc = devclass_get_softc(acpi_devclass, 0)) == NULL)
+	panic("can't find ACPI device to register interrupt");
+    if (sc->acpi_dev == NULL)
+	panic("acpi softc has invalid device");
+
     if ((InterruptNumber < 0) || (InterruptNumber > 255))
 	return_ACPI_STATUS(AE_BAD_PARAMETER);
     if (ServiceRoutine == NULL)
@@ -67,11 +72,6 @@ AcpiOsInstallInterruptHandler(UINT32 InterruptNumber, OSD_HANDLER ServiceRoutine
     }
     InterruptHandler = ServiceRoutine;
 
-    if ((sc = devclass_get_softc(acpi_devclass, 0)) == NULL)
-	panic("can't find ACPI device to register interrupt");
-    if (sc->acpi_dev == NULL)
-	panic("acpi softc has invalid device");
-
     /*
      * This isn't strictly true, as we ought to be able to handle > 1 interrupt.  The ACPI
      * spec doesn't call for this though.