OpenBLAS

cpp_thread_test/dgemv: fail early if concurrency is zero

The two test cases dgemv_tester and dgemm_tester accept the degree of
concurrency as command line argument (amongst others). Fail early if
value 0 has been specified, instead of later with less-clear symptoms.

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>