Null out mjData.buffer and mjData.stack before possibly calling mj_deleteData.

While creating an mjData, any error triggers mj_deleteData. If buffer or stack is not NULL at that point, an uninitialized pointer will be freed.

PiperOrigin-RevId: 465378329
Change-Id: I9be0eef0648e05e3e5f1361da346a5046485cde7

src/engine/engine_io.c

@@ -838,6 +838,7 @@ static mjData* _makeData(const mjModel* m) {
 
   // compute buffer size
   d->nbuffer = 0;
+  d->buffer = d->stack = NULL;
 #define X(type, name, nr, nc)                                                \
   if (!safeAddToBufferSize(&offset, &d->nbuffer, sizeof(type), m->nr, nc)) { \
     mju_warning("Invalid data: " #name " too large.");                       \
@@ -871,7 +872,9 @@ static mjData* _makeData(const mjModel* m) {
 
 mjData* mj_makeData(const mjModel* m) {
   mjData* d = _makeData(m);
-  mj_resetData(m, d);
+  if (d) {
+    mj_resetData(m, d);
+  }
   return d;
 }