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.

tests/commandtest.c

@@ -690,6 +690,7 @@ static int test17(const void *unused ATTRIBUTE_UNUSED)
         goto cleanup;
     }
 
+    sa_assert(outbuf);
     if (*outbuf) {
         puts("output buffer is not an allocated empty string");
         goto cleanup;