Fix SBGEMM unit test to handle zero elements.

[ChipKerchner]

Fix SBGEMM unit test to handle zero elements - since malloc zero bytes fails on some systems.

[ChipKerchner]

Fixes malloc(0) failing for AIX.

[martin-frbg]

Hmm, that's annoying. M,N,K equal 0 might be a valid corner case to test, though not that important.

[ChipKerchner]

Fixed to handle zero elements.

[ChipKerchner]

Actually zero length GEMV will not work because of this (and other checks)

if (lda < MAX(1, m))   info = 6;

If it is included, I get this

 ** On entry to SGEMV  parameter number  6 had an illegal value
 ** On entry to SBGEMV  parameter number  6 had an illegal value

[ChipKerchner]

So this PR is ready. Sorry about all the changes.

[martin-frbg]

Thank you (sorry for causing extra work with what was just a spontaneous thought before going on a long drive)