Added test for servo#353 and fixed use after free bug that causes it #354
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I took a look at what was causing #353.
Turns out the
push_heapfunction was placing the heap pointer into a local variable before callingself.reserve(). reserve in turn callsrealloc()- which can free the old heap allocation & create a new one. When this happens, the oldptris becomes invalid.The code was writing to this invalid pointer (
ptr.as_ptr().add(len).write(value)) - which is a use-after-free bug.This PR fixes the bug and adds a regression test.