Fix brotlidump.py crashing when complex prefix code has exactly 1 non-zero code length (#635)

According to the format specification regarding complex prefix codes:

> If there are at least two non-zero code lengths, any trailing zero
> code lengths are omitted, i.e., the last code length in the
> sequence must be non-zero.  In this case, the sum of (32 >> code
> length) over all the non-zero code lengths must equal to 32.

> If the lengths have been read for the entire code length alphabet
> and there was only one non-zero code length, then the prefix code
> has one symbol whose code has zero length.

The script does not handle a case where there is just 1 non-zero code
length where the sum rule doesn't apply, which causes a StopIteration
exception when it attempts to read past the list boundaries.

An example of such file is tests/testdata/mapsdatazrh.compressed. I made
sure this change doesn't break anything by processing all *.compressed
files from the testdata folder with no thrown exceptions.