fix case #56: Text builder instance writes to wrong offset

[goertzenator]

Fixes case #56 , Text builder instance writes to wrong offset.

The added test case without the fix gives the failure below. The linked case gives a better sense of what was going wrong here.

Running 1 test suites...
Test suite tests: RUNNING...
Properties:
  b_shortest: [OK, passed 100 tests]
  t_shortest: [OK, passed 100 tests]
  catTextEqualsCatBuilder: [Failed]
*** Failed! Falsified (after 3 tests and 2 shrinks):
[Right 0.0,Right 0.0]
(used seed 3471324215185070192)
  catByteStringEqualsCatBuilder: [OK, passed 100 tests]
Regressions:
  toShortest_overflow: [OK]

         Properties  Test Cases  Total
 Passed  3           1           4
 Failed  1           0           1
 Total   4           1           5
Test suite tests: FAIL