memshare: Avoid accessing uninitialized nodes

Prevent iterating over uninitialized memshare child nodes
while handling client alloc and free requests.

Change-Id: I421cd239bb999176e587ab0c06757d83485fad6e
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>
Signed-off-by: Meenu Raja Sundaram <quic_mrajasun@quicinc.com>