thinko in convertToJsonb()

The call:

reserveFromBuffer(&buffer, sizeof(VARHDRSZ))

is assuming that the size of varlena header is the same
size as the type used to return that size, which happens
to be so, but someone could easily change that macro
to:

#define VARHDRSZ ((int64) sizeof(int32))

And you'd want to reserve sizeof(int32), not sizeof(int64)
in convertToJsonb.

Perhaps the code really meant to say:

reserveFromBuffer(&buffer, VARHDRSZ)

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