bring the two loops in act_txtobj into alignment

The function act_txtobj() performs two passes over the sequence of
text operators. The first pass tracks position and length of the
string. Then an array is allocated of the indicated length and a
second pass fills it with the actual characters.

The two passes must be close carbon copies of each other or a mismatch
between the predetermined length and the actual number of characters
produced might occur and cause the assertion "txtlen == idx" to fail.

This code structure is obviously bad, a text book example of why code
duplication should be avoided. Nevertheless, before rewriting it
entirely, this patch at least corrects an immediate bug.

Fixes #44.