dac960: fix undefined behavior on empty string

Fix undefined behavior due to a buffer underrun if an empty string is
written to the proc file.
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/DAC960.c

@@ -6562,7 +6562,7 @@ static int DAC960_ProcWriteUserCommand(struct file *file,
   if (copy_from_user(CommandBuffer, Buffer, Count)) return -EFAULT;
   CommandBuffer[Count] = '\0';
   Length = strlen(CommandBuffer);
-  if (CommandBuffer[Length-1] == '\n')
+  if (Length > 0 && CommandBuffer[Length-1] == '\n')
     CommandBuffer[--Length] = '\0';
   if (Controller->FirmwareType == DAC960_V1_Controller)
     return (DAC960_V1_ExecuteUserCommand(Controller, CommandBuffer)