Slop in shrunk arrays not zero'd when using -hT
The heap profiler be used even in a non-profiled RTS via its
+RTS -hT mode. However, the
OVERWRITING_CLOSURE_MUTABLE macro used when shrinking arrays is a no-op in the non-profiled RTS. Consequently, we can end up with non-zero'd slop which may lead to unsound behavior during heap census.
I noticed this while working on #7275.