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>

util.c

@@ -152,7 +152,6 @@ char get_escape_char(const char *s, int *i)
 	int	j = *i + 1;
 	char	val;
 
-	assert(c);
 	switch (c) {
 	case 'a':
 		val = '\a';