postgresql: avoid to crash with numerical passwords

Issue: https://github.com/sosreport/sos/issues/433
Change-Id: I29d0109dcf053387203fe7e10f45b68d8543c5ff
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1133993
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>