important safety check in SlowControlCollection.h

[marc1uk]

Check requested control exists in templated GetValue method.

Note that this simply returns a default-constructed value, but does not communicate to the user that the requested control was not found. This could lead to some subtle and hard-to-find bugs if typos are made in Get calls. :(

Similar behaviour is affected by the SlowControlElement class GetValue method.

[marc1uk]

Alejandro believes his error may not be from a stray templated Get call generating an invalid entry in SC_vars, but rather due to memory exhaustion from creating too many SlowControlElements. Pretty unlikely on a server/desktop, but in embedded environments I suppose it's possible.
Would require an additional check on SlowControlCollection::Add for new calls that return nullptr, erasing the corresponding map entry. Not sure how we feed either of these errors back to the user, though.