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See https://bugzilla.redhat.com/show_bug.cgi?id=692632 for more details and a proposed patch.

IIUC, this comes up with invalid po files, so this report is about graceful error handling, not strictly a bug.
If https://bugzilla.redhat.com/attachment.cgi?id=489427 was applied, gettext would not fail anymore, but the invalid line would not be reported IIUC. I think we should print an error message, to let libs/apps find the mistake.

Here's a patch against 3.5. This patch is atop the fix for #17898, so it won't apply cleanly to a stock 3.5.
The patch uses warnings.warn() to issue a warning if the plural header is empty. There seems to be precedent for issuing warnings in the case of odd incoming data or behaviour, so I'm comfortable doing this; I think that warnings need not be only for Python language features, like deprecated things.
But this does mean that importing gettext will also import warnings as a result, increasing memory usage. Probably this doesn't matter -- on any sizable program, you probably already have the warnings module imported. Otherwise we could print to sys.stderr.

LGTM

Sorry, I don't see a value of issuing warning if the following line of the code is failed. Why not raise an exception instead?
A warning can be issued in the case of lacking nplurals= which is not used in Python implementation.
I think additional checks should be added in msgfmt.py. GNU msgfmt checks that plural= and nplurals= are specified if there are plural form translations.