lib/parser: fix month days

In the olden days, we stored dates like e.g. 20180131121234, and do a
lexicographical check. With that, we could use e.g. upper-limits
201802312359 for "all dates in Feb 2018", even if Feb doesn't have 31
days.

However, nowadays we use time_t values, and g_date_time_new_local raises
errors for non-existent days; easiest fix is to massage things a bit; so
let's do that.

Fixes issue #1197.

lib/parser/test-utils.cc

@@ -62,6 +62,9 @@ test_date_basic ()
 		{ "1972-12-14T09:10:23", true,	"0093165023" },
 		{ "1854-11-18T17:10:23", true,	"0000000000" },
 
+		{ "2000-02-31T09:10:23", true,  "0951861599" },
+		{ "2000-02-29T23:59:59", true,  "0951861599" },
+
 		{ "2016",		true,	"1451599200" },
 		{ "2016",		false,  "1483221599" },