Fix GH-16261: Reference invariant broken in mb_convert_variables()

The behaviour is weird in the sense that the reference must get
unwrapped. What ended up happening is that when destroy

Fix GH-16261: Reference invariant broken in mb_convert_variables()

The behaviour is weird in the sense that the reference must get
unwrapped. What ended up happening is that when destroying the old
reference the sources list was not cleaned properly. We add handling for
that. Normally we would use use ZEND_TRY_ASSIGN_STRINGL but that doesn't
work here as it would keep the reference and change values through
references (see bug #26639).

Closes GH-16272.

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