The original coding examined the next character before verifying that
there *is* a next character.  In the worst case with the input buffer
right up against the end of memory, this would result in a segfault.

Problem spotted by Paul Guyot; this commit extends his patch to fix an
additional case.  In addition, make the code a tad more readable by not
overloading the usage of *tlen.

provided by Andrew.

avoid this problem in the future.)

hard-to-maintain textual examples currently in the SGML docs.  From
Sergey Karpov.