vchoi_fork

[svn-r21221] The original code 
  i_mask = ~((unsigned)~0 << (precision[0] + offset[0])) & ((unsigned)~0 << offset[0]);
in line 3680 left shifted a 32-bit integer for 32-bit.  The result is undefined by C language.  A user
discovered it using clang compiler with -fcatch-undefined-behavior option (see Issue 7674
in Jira).  So I changed it in a funny way to avoid it.

Tested on jam.  But I tested the same change for the trunk with h5committest.