of: Fix crash if an earlycon driver is not found

__earlycon_of_table_sentinel.compatible is a char[128], not a pointer, so
it will never be NULL.  Checking it against NULL causes the match loop to
run past the end of the array, and eventually match a bogus entry, under
the following conditions:

 - Kernel command line specifies "earlycon" with no parameters
 - DT has a stdout-path pointing to a UART node
 - The UART driver doesn't use OF_EARLYCON_DECLARE (or maybe the console
   driver is compiled out)

Fix this by checking to see if match->compatible is a non-empty string.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: <stable@vger.kernel.org> # 3.16+
Signed-off-by: Rob Herring <robh@kernel.org>