fix: memory leaks when converting data to C

[szhorvat]

Attempted fix for #1173 (comment)

CC @Antonov548, not sure I covered everything.

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[szhorvat]

@krlmlr Can we please have this in phoenix, and include it in the Tuesday release, without the pressure to spend time on extra tests? This fixes some memory leaks of the type discovered in #1173 (comment)

I untangled the manual changes in rinterface.c from the ones due to this bugfix. Hopefully I didn't make a mistake.

[krlmlr]

Thanks!