Timer 1 COMPA ISR - default behaviour?
The compiler cannot detect a missing ISR since it is very difficult, in
the general case, to tell whether any particular interrupt-enable bit
has been set by the program.

The avr-libc library does indeed provide a “catch-all” ISR and, as
stated by the manual,


  the default action is to reset the device by jumping to the reset
  vector.


If you don't like this behavior, you can override this catch-all with
your own, as explained in the manual (search for “Catch-all
interrupt vector”).