Add missing loop over block columns of C

[angsch]

The block column width of C is defined by #define nb 1000 and used to allocate the buffers, but otherwise nb is unused. As a result, if the m-by-n matrix C has n > nb, the code encounters a segmentation fault. This commit fixes this issue by

• adding the missing loop over the block columns of C, and
• adjusting the default leading dimensions to whatever the row count is.