BibFormat: max_char adjustment in bfe_abstract

Problem:
Current implementation using the max_char parameter might cut the abstract in a mathematical formula, thus making it impossible to render.

Solution:
Basically interpreting each formula as a string with the length 1, so they cannot be cut in the middle.
This is done with the extend_max_chars function, which increases the max_char parameter by the length of the formulas in case the abstract contains some.