Properly handle embedded nul delimited string lists

For example:

reserved-names="res1\0res2\0res3";

Where \0 is an actual embedded NUL in the source instead of a string
escape. To achieve this, use the len given by the lexer instead of
strlen.

Without this patch dtc will mangle the output and possibly hang on
realloc.

data.c

@@ -74,7 +74,7 @@ struct data data_copy_escape_string(const char *s, int len)
 	struct data d;
 	char *q;
 
-	d = data_grow_for(empty_data, strlen(s)+1);
+	d = data_grow_for(empty_data, len + 1);
 
 	q = d.val;
 	while (i < len) {