hdf5_msb

[svn-r19798] Purpose:

    Add additional error checking to catch erroneous user input.

Description:

    Attempting to retrieve a links's name by index in the case
    where the link is external and the file that the object is
    located in doesn't exist was causing a segmentation fault
    (in production) and an assertion failure (in debug).

    The segfault wasn't occuring until the metadata accumulator
    attempted a write, so I've added error checking higher in
    the pipeline in H5O_protect (where there was previously just
    an assert) to catch this. I've also added additional asserts
    in the H5F layer where there were none.

    Additionally, I added another case to the links.c test to
    test that this fails gracefully instead of segfaulting or 
    asserting out.

Tested:

    h5committest and gandalf (mac os x)