Coverity: Resolve a FORWARD_NULL
    
    Recent changes in the module seemed to have caused Coverity to reanalyze
    certain parts of the code. Previously the code was modified via commit
    id '11a11812' to resolve a different error (perhaps DEADCODE).  Up through
    commit id '7b3f1f8c' there were no issues.
    
    The new error indicats the 'outbuf' was checked for NULL and then complains
    because of the dereference. Adding checks for non-NULL prior to the deref
    resulted in a DEADCODE message.
    
    So, resolve using an sa_assert() to keep Coverity quiet especially since
    it doesn't understand that outbuf will change as a result of a successful
    virCommandRun() call.