fix two other potential issues with leftover 32 bit integers when reading from sndfil...

fix two other potential issues with leftover 32 bit integers when reading from sndfilesource.

It is less likely that these would cause issues because the
variables involved define the size of the data read, which
is almost certainly less than the 32 bit limit (i.e. they are
not positional). But to keep things clean and to keep questions
at bay, make them 64 bit values.