msm:cpp: Check number of buffers before allocate stream_info

Number of buffers used by cpp is defined by user space. In
case this number is ZERO the allocated memory space will be
pointed by ZERO_SIZE_PTR. Referencing ZERO_SIZE_PTR, causes
same type of behavior as referencing the NULL pointer

Change-Id: I42665225c7d055cda6749198c35e94c1442c6d3e
Signed-off-by: Iliya Varadzhakov <ivarad@codeaurora.org>