Resolve USAN in basop32.c

This is related to #1528 (closed), where it is found that L_add_o, L_add_co, shl_o and L_sub have an undefined behavior. This is a fix to resolve this, which should probably be propagated to all our code bases and to ITU-T STL. It is expected to be bit exact -- the undefined operation seems to do what we expect.

Pinging @multrus and @doehla for review.