hdf5_ep

[svn-r23499] Merge from trunk

Fixes an issue (HDFFV-8159) where uninitialized memory was being read
during variable-length type conversion.  All variable-length type conversion
buffers are now initialized to contain 0s on allocation.

Tested on: jam w/ fortran and c++

This is a very small change (basially changing malloc to calloc) and
is unlikely to cause issues on other platforms.