Fix minor thinko in convertToJsonb().

The amount of space to reserve for the value's varlena header is
VARHDRSZ, not sizeof(VARHDRSZ).  The latter coding accidentally
failed to fail because of the way the VARHDRSZ macro is currently
defined; but if we ever change it to return size_t (as one might
reasonably expect it to do), convertToJsonb() would have failed.

Spotted by Mark Dilger.