[patch] prevent possible NULL dereference.

[Dan Carpenter <error27@gmail.com>]

Move the check for walk_state->thread before the dereference.

Found by smatch static checker.  Compile tested.

regards,
dan carpenter

Signed-off-by: Dan Carpenter <error27@gmail.com>

--- orig/drivers/acpi/acpica/exmutex.c	2009-11-07 09:46:53.000000000 +0200
+++ devel/drivers/acpi/acpica/exmutex.c	2009-11-07 09:48:16.000000000 +0200
@@ -375,6 +375,15 @@
 		return_ACPI_STATUS(AE_AML_MUTEX_NOT_ACQUIRED);
 	}
 
+	/* Must have a valid thread ID */
+
+	if (!walk_state->thread) {
+		ACPI_ERROR((AE_INFO,
+			    "Cannot release Mutex [%4.4s], null thread info",
+			    acpi_ut_get_node_name(obj_desc->mutex.node)));
+		return_ACPI_STATUS(AE_AML_INTERNAL);
+	}
+
 	/*
 	 * The Mutex is owned, but this thread must be the owner.
 	 * Special case for Global Lock, any thread can release
@@ -392,15 +401,6 @@
 		return_ACPI_STATUS(AE_AML_NOT_OWNER);
 	}
 
-	/* Must have a valid thread ID */
-
-	if (!walk_state->thread) {
-		ACPI_ERROR((AE_INFO,
-			    "Cannot release Mutex [%4.4s], null thread info",
-			    acpi_ut_get_node_name(obj_desc->mutex.node)));
-		return_ACPI_STATUS(AE_AML_INTERNAL);
-	}
-
 	/*
 	 * The sync level of the mutex must be equal to the current sync level. In
 	 * other words, the current level means that at least one mutex at that