aborting C gse2 decompression when n_samps == 0

reading gse2 files with n_samps = 0 will directly lead to segmentation
faults. Therefore, raising an Exception if that happens (see #538).
Even better handling for corrupt gse2 header.

Let the checksum detect the error when applied to a zero length array,
that's what a checksum is for, see #538.
correcting test file name

see #538
Merge pull request #538 from mbyt/538

obspy.core.read() no return from function when GSE2 chksum is wrong