ASoC: dsp: validate widget list pointer before using it

There is a possiblity that the number of valid path is greater
than 0 even though widget list is completely empty. This condition
causes the data abort in dsp_add_new_paths(). Fix the issue by
exiting the function when widget list is empty.

Change-Id: I161e87f2eba61ac383fe83641206652b9cf217f7
CRs-fixed: 329351
Signed-off-by: Jay Wang <jaywang@codeaurora.org>