Sansa AMS: identify interrupts with no source set

They would previously appear like coming from the (disabled) watchdog
module

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@22251 a1c6a512-1295-4272-9138-f99709370657

firmware/target/arm/as3525/system-as3525.c

@@ -87,6 +87,10 @@ static void UIRQ(void)
 {
     unsigned int irq_no = 0;
     int status = VIC_IRQ_STATUS;
+
+    if(status == 0)
+        panicf("Unhandled IRQ (source unknown!)");
+
     while((status >>= 1))
         irq_no++;