checkpatch: handle missing #if open in context

If the #if opening statement is not in the context then the context stack
can be empty.  Handle this by ensuring there is always a blank entry in
the stack.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Tested-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>