Cope with argv[0] being null in the dynamic linker.

Somewhat unsurprisingly, very few commands are happy to be run like this,
in particular multiplexed commands like toybox. But that's no reason for
the linker to get in the way too.

Bug: http://b/33276926
Test: new test
Change-Id: I6dd71ea0183f4da83571039c2198ebb6ed38520e