Add missing return statement

[liubovch]

Hi! I was tracking down an error that occurred during the build of SNAP inside Debian:

    LandauVishkinTest:
    - equal strings: [OK]
    - prefixes: [OK]
    - non-equal strings: [OK]
    - overly distant strings: [OK]
    - CIGAR strings: [OK]

    EventTest:
    - many waiters: make[1]: *** [debian/rules:17: override_dh_auto_test] Segmentation fault

I have found that there is missing return that caused the segfault:

SNAPLib/Compat.cpp: In member function ‘bool SingleWaiterObjectImpl::destroy()’:
SNAPLib/Compat.cpp:1122:5: warning: no return statement in function returning non-void [-Wreturn-type]
     }
     ^

[AmplabJenkins]

Can one of the admins verify this patch?