Part of bug #580212 - Report inline parts only when found both tags

When parsing inline parts, like inline signatures, report the part
type only when found also the end tag for the part type.

mail/em-inline-filter.c

@@ -325,6 +325,9 @@ emif_scan(CamelMimeFilter *f, gchar *in, size_t len, gint final)
 	}
 
 	if (final) {
+		/* always stop as plain, especially when not read those tags fully */
+		emif->state = EMIF_PLAIN;
+
 		emif_add_part(emif, data_start, inend-data_start);
 	} else {
 		g_byte_array_append(emif->data, (guchar *)data_start, inend-data_start);