reserve enough UID ranges in imap_load_box()

in certain configurations, under very unlikely conditions (which are
practically impossible to control remotely), we'd overflow ranges[].
in a typical gcc build, the values (which are also practically
impossible to control remotely) would be written at the end of buf[],
which would be rather harmless, as only a tiny part of buf is used
subsequently. so i'm not classifying this as a security issue.

amends 77acc268.

src/drv_imap.c

@@ -2883,7 +2883,7 @@ imap_load_box( store_t *gctx, uint minuid, uint maxuid, uint finduid, uint pairu
 		if (maxuid == UINT_MAX)
 			maxuid = ctx->uidnext - 1;
 		if (maxuid >= minuid) {
-			imap_range_t ranges[3];
+			imap_range_t ranges[4];
 			ranges[0].first = minuid;
 			ranges[0].last = maxuid;
 			ranges[0].flags = 0;