Fix bug in -Odts with properties containing multiple terminating nulls

When in -Odts mode, dtc will not produce correct output for
string-like properties which have more than one \0 character at the
end of the property's bytestring.  In fact, it generates output which
is not syntactically correct.  This patch fixes the bug, and adds a
testcase for future regressions here.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/extra-terminating-null.dts

@@ -0,0 +1,11 @@
+/dts-v1/;
+
+/ {
+	extranull0 = "hello world";
+	extranull1,1 = "hello world\0";
+	extranull1,2 = "hello world", "";
+	extranull2,1 = "hello world\0\0";
+	extranull2,2 = "hello world", "", "";
+	extranull2,3 = "hello world\0", "";
+	extranull2,4 = "hello world", "\0";
+};