OpenBLAS

Fix division by zero in zrotg

The cases
[ c  s ] * [ 0      ] = [ |db_i| ]
[-s  c ]   [ i*db_i ]   [  0     ]
and
[ c  s ] * [ 0      ] = [ |db_r| ]
[-s  c ]   [ db_r   ]   [  0     ]
computed s incorrectly. To flip the entries of vector,
s should be conjg(db)/|db| and not conjg(db) / da,
where da == 0.0.