hdf5_ep

Fix HDFFV-11120 and HDFFV-11121 (CVE-2018-13870 and CVE-2018-13869)

Description:
    When a buffer overflow occurred because a name length was corrupted
    and became very large, h5dump produced a segfault on one file and a
    memcpy parameter overlap on another file.  This commit added checks
    that detect a read pass the end of the buffer to prevent these error
    conditions.
Platforms tested:
    Linux/64 (jelly)
    SunOS 5.11 (emu)