Fix for [perl #9423] vec assignments generate 2 warnings

Fix for [perl #9423] vec assignments generate 2 warnings

Before this commit, this:

    vec(my $v,0,1) = 1;

would've produced four warnings about uninitialized values;
however, the ticket argued that these were spurious.

This commit removes the warning in the case of lvalue vec, since it is
similar to |=, but leaves it in place for rvalue vec.