planner: fix C interface reduce_span return code

Problem: there is a bug in the planner C API function to reduce a span.
The final condition has a typo in the return code computation that can
cause incorrect behavior.

Fix the bug.

resource/planner/c/planner_c_interface.cpp

@@ -628,9 +628,8 @@ extern "C" int planner_reduce_span (planner_t *ctx,
         update_mintime_resource_tree (ctx, list);
         rc = 0;
     } else {
-        // Error
+        // Error; rc already -1
         errno = EINVAL;
-        rc - 1;
     }
 
     return rc;