opus: Fix typo causing overflow in silk_stabilize_lsf

Due to this typo max_center can be too large, causing nlsf to be set to
too large values, which in turn can cause nlsf[i - 1] + min_delta[i] to
overflow to a negative value, which is not allowed for nlsf and can
cause an out of bounds read in silk_lsf2lpc.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
