Fix crash on nul character in string escape sequence

If a dts file contains a string with \ followed by a nul byte - an actual
nul in the input file, not the \\0 escape - then the assert() in
get_escape_char() will trip, crashing dtc.

As far as I can tell, there isn't any valid need for this assert(), so just
remove it.

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>