Improve test coverage

[GoogleCodeExporter]

Code coverage of the test has fallen to 84%:
http://lakka.kapsi.fi:50140/job/nanopb%20code%20coverage/lastSuccessfulBuild/art
ifact/tests/build/coverage/index.html

A lot of the uncovered lines are error propagation if():s. Simulating IO 
failures would be a good way to test the code for correct error propagation, 
even though it is already checked using GCC's warning system.

Also there is some actual code that is not covered by the tests currently. This 
should be fixed by adding relevant test cases.

Original issue reported on code.google.com by Petteri.Aimonen on 20 Jul 2014 at 12:28

[GoogleCodeExporter]

This issue was updated by revision d0299d87ec10.

Original comment by Petteri.Aimonen on 7 Sep 2014 at 5:32

• Changed state: Started

[GoogleCodeExporter]

This issue was updated by revision 8189d538dd80.

Original comment by Petteri.Aimonen on 7 Sep 2014 at 5:32

• Changed state: FixedInGit

[GoogleCodeExporter]

Fix released in nanopb-0.3.1

Original comment by Petteri.Aimonen on 11 Sep 2014 at 5:13

• Changed state: FixReleased