Rewrite last lines of matrix_exp.hpp using ? operator instead of an if/else statement.
Rewrite last lines of matrix_exp.hpp using ? operator instead of an if/else statement.

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math