dsp: q6core: Avoid OOB access in q6core

"num_services", a signed integer when compared
with constant results in conversion of signed integer
to max possible unsigned int value when "num_services"
is a negative value. This can lead to OOB read.
Fix is to handle this case.

Change-Id: Id6a8f150d9019c972a87f789e4c626337a97bfff
Signed-off-by: Soumya Managoli <quic_c_smanag@quicinc.com>