fix issue with negative "all archives" size, fixes #165
fix issue with negative "all archives" size, fixes #165

This fixes a infrequent problem when (refcount * chunksize) overflowed a int32_t.
chunksize is always <= 8MiB and usually rather ~64KiB (with default chunker params).
Thus, this happened only for high refcounts and/or unusually big chunks.

RonnyPfannschmidt/borg,edgimar/borg,edgewood/borg,RonnyPfannschmidt/borg,edgimar/borg,level323/borg,edgimar/borg,edgewood/borg,level323/borg,edgimar/borg,edgewood/borg,RonnyPfannschmidt/borg,edgewood/borg,mhubig/borg,raxenak/borg,RonnyPfannschmidt/borg,raxenak/borg,RonnyPfannschmidt/borg,raxenak/borg,level323/borg,mhubig/borg,mhubig/borg,raxenak/borg