Bluetooth: Mesh: Add buf size check in comp p1 get

According to mesh 1.1 spec (4.4.1.2.2 Composition Data state):
“If the complete list of models does not fit in the Data field, the
element shall not be reported.“

The implementation of bt_mesh_comp_data_get_page_1 does
not consider this, and further it will assert the device if the passed
buffer is to small.

This commit adds a check that prevents populating passed buffer
if the full element entry wont fit in the buffer.

Signed-off-by: Anders Storrø <anders.storro@nordicsemi.no>