Fix one-byte buffer overrun in contrib/test_parser.
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.