Reset contact efc_address in mj_clearEfc, fixes #2869

The truth is that all these solver-related values should be moved out of `mjContact` altogether, but that can be done in a future followup.

PiperOrigin-RevId: 818624976
Change-Id: I5b9e63927098cdf26b9f8acfdd9d86e75d6ce331

src/engine/engine_memory.h

@@ -75,6 +75,11 @@ static inline void mj_clearEfc(mjData* d) {
   d->nefc = 0;
   d->nisland = 0;
   d->contact = (mjContact*) d->arena;
+
+  // if any contacts are allocated, clear their efc_address
+  for (int i=0; i < d->ncon; i++) {
+    d->contact[i].efc_address = -1;
+  }
 }
 
 #ifdef __cplusplus