UBSan spotted that |src->Basic.kind| had a value outside the range of
|BasicKind| due to it actually being a |Type_Pointer|. Since these are
stored in a union there could be cases where the value of |kind| just
so happens to be |Basic_string|, in which case the branch would have
been taken when it shouldn't have been.
To fix this simply check that it's a |Type_Basic| before treating it as
a |Basic|.
