[GC] Cleared the metadata attached to a PMC_EXT structure when recycling it. This prevents stale (and themselves recycled) metadata PMCs from sticking around nothing expects them.  This fixes at least RT #55782.
[GC] Cleared the metadata attached to a PMC_EXT structure when recycling it.
This prevents stale (and themselves recycled) metadata PMCs from sticking
around nothing expects them.  This fixes at least RT #55782.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@28836 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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