Hmm, so actually most of the bugs fixed in 2.7 and 3.2 weren't present
in 3.3 and 3.4, and those versions already had tests equivalent to the
tests I added for 2.7/3.2.

As for the changes that I did make to 3.3/3.4:

- two of the three cover cases that only occur if the output data is
  larger than ~32GiB. Even if we have a buildbot with enough memory for
  it (which I don't think we do), actually running such tests would take
  forever and then some.

- the third is for a condition that's actually pretty much impossible to
  trigger - grow_buffer() has to be called on a buffer that is already at
  least 8*((size_t)-1)/9 bytes long. On a 64-bit system this is
  astronomically large, while on a 32-bit system the OS will probably
  have reserved more than 1/9th of the virtual address space for itself,
  so it won't be possible to allocate a large enough buffer.