shared/gatt-db: Fix scan-build warnings

shared/gatt-db: Fix scan-build warnings

This fixes the following warnings:

src/shared/gatt-db.c:1339:2: warning: Undefined or garbage value
returned to caller [core.uninitialized.UndefReturn]
        return data.num_of_res;
        ^~~~~~~~~~~~~~~~~~~~~~

src/shared/gatt-db.c:725:5: warning: Access to field 'handle' results
in a dereference of a null pointer
        service->attributes[0]->handle == handle)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>