Debian package: fix warning during preconfiguration.

debian/changelog

@@ -1,3 +1,9 @@
+matrix-synapse-py3 (0.99.3) UNRELEASED; urgency=medium
+
+  * Fix warning during preconfiguration. (Fixes: #4819)
+
+ -- Richard van der Hoff <richard@matrix.org>  Thu, 07 Mar 2019 07:17:00 +0000
+
 matrix-synapse-py3 (0.99.2) stable; urgency=medium
 
   * Fix overwriting of config settings on upgrade.

debian/matrix-synapse-py3.config

@@ -5,7 +5,11 @@ set -e
 . /usr/share/debconf/confmodule
 
 # try to update the debconf db according to whatever is in the config files
-/opt/venvs/matrix-synapse/lib/manage_debconf.pl read || true
+#
+# note that we may get run during preconfiguration, in which case the script
+# will not yet be installed.
+[ -x /opt/venvs/matrix-synapse/lib/manage_debconf.pl ] && \
+    /opt/venvs/matrix-synapse/lib/manage_debconf.pl read
 
 db_input high matrix-synapse/server-name || true
 db_input high matrix-synapse/report-stats || true