Fix GH-15908 and GH-15026: leak / assertion failure in streams.c

This was first reported as a leak in GH-15026, but was mistakingly
believed to be a false positive. Then an assertion was

Fix GH-15908 and GH-15026: leak / assertion failure in streams.c

This was first reported as a leak in GH-15026, but was mistakingly
believed to be a false positive. Then an assertion was added and it got
triggered in GH-15908. This fixes the leak. Upon merging into master the
assertion should be removed as well.

Closes GH-15924.

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