For s///r, don't call SvPV_force() on the original value. Resolves #97954.

For s///r, don't call SvPV_force() on the original value. Resolves #97954.

8ca8a454f60a417f optimised the implementation of s///r by avoiding an
unconditional copy of the original value. However, it introduced a behaviour
regression where if original value happened to be one of a few particular
types, it could be modified by being forced to a string using SvPV_force().
The substitution was (correctly) performed on a copy of this string.