gsw_C_from_SP should check that 0 <= SP <= 42

[richardsc]

Discussed originally for the TEOS-10/GSW-C#48, the idea is to update the following in gsw_C_from_SP() to restrict to the range of 0 to 42:

% This line ensures that SP is non-negative.
if any(SP < 0)
    error('gsw_C_from_SP: SP must be non-negative!')
end