pybgpstream fields parsing error for large AS path field

[digizeph]

pybgpstream segfaults when extracting elements for updates with very large AS path field (larger than 4096 characters). The issue is caused by an assumption made in AS path field parsing that the AS path field would not be larger than 4k characters.

[digizeph]

This issue is addressed in #81 where the program will dynamically allocate larger memory if necessary when the AS path field is very large.
#81 also fixed a number of issues in AS path parsing that allows the program to write \0 to the locations outside the buffer.