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.

research/brotlidump.py

@@ -1467,16 +1467,17 @@ class Layout:
         #we use it for display until now; definition comes below
         lengthCode = LengthAlphabet('#'+alphabet.name)
         lengthIter = iter(lengths)
-        while total<32:
+        lengthsLeft = len(lengths)
+        while total<32 and lengthsLeft>0:
+            lengthsLeft -= 1
             newSymbol = next(lengthIter)
             lol.description = str(lengthCode[newSymbol])
             length = self.verboseRead(lol)
             if length:
                 codeLengths[newSymbol] = length
                 total += 32>>length
-            if total>=32:
-                break
         if total>32: raise ValueError("Stream format")
+        if len(codeLengths)==1: codeLengths[list(codeLengths.keys())[0]] = 0
         #Now set the encoding of the lengthCode
         lengthCode.setLength(codeLengths)
         print("***** Lengths for {} will be coded as:".format(alphabet.name))