src/pmlogsize/data.c: change buffer management strategy
src/pmlogsize/data.c: change buffer management strategy

Change from high-watermark malloc() scheme to __pmFindPDUBuf().

The buffer pin/unpin protocol for the native libpcp PDU buffers
works better with __pmDecodeResult() that is used here.

Fixes a bunch of QA failures on some platforms.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp