Resolve "ProcessorLoopClosureBase class"

Closes #220 (closed)

src/processor/processor_loopclosure_base.cpp

@@ -34,8 +34,16 @@ void ProcessorLoopClosureBase::process(CaptureBasePtr _incoming_ptr)
         {
             CaptureBasePtr capture_1 = pairKC.second;
             CaptureBasePtr capture_2 = findLoopCandidate(capture_1);
-            if (capture_2==nullptr) return;
-            if (validateLoop(capture_1, capture_2)==false) return;
+            if (validateLoop(capture_1, capture_2)==false)  return;
+            if (capture_2==nullptr)                         return;
+            if (capture_1->getFrame() == nullptr || capture_2->getFrame() == nullptr) {
+                WOLF_WARN("ProcessorLoopClosureBase : tried to close a loop with captures linked to no KF");
+                return;
+            };
+            if (capture_1->getFrame() == capture_2->getFrame()) {
+                WOLF_WARN("ProcessorLoopClosureBase : findLoopCandidate() returned two captures of the same frame");
+                return;
+            };
             createFactors(capture_1, capture_2);
         };
     };