lib/pattern: Support NULL output buffer in parse_and_fill_pattern()

lib/pattern: Support NULL output buffer in parse_and_fill_pattern()

Support passing a NULL output buffer to parse_and_fill_pattern().
Each formatting function simply needs to avoid accessing the buffer
when it is NULL. This allows calculating the required size of the
buffer before one might be allocated.

This will be useful in a subsequent patch for dynamically allocating
the pattern buffers.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>