BUG: special/specfun: fix bug in MTU12

Off-by-one error in the order up to which the Bessel functions were
computed (the arrays are accessed up to KM+1). Doesn't seem to affect
results much as the series seems mostly to converge before reaching the
limit.

Covered by test_mathieu_modsem2 (see Trac #1847)

(backport of 8bcdd37)