Fix (unreported) potential leak in Main freeing function.

Could happen in case a 'split-by-libraries' Main is passed to
`BKE_main_free`.

source/blender/blenkernel/intern/main.c

@@ -40,6 +40,13 @@ Main *BKE_main_new()
 
 void BKE_main_free(Main *mainvar)
 {
+  /* In case this is called on a 'split-by-libraries' list of mains.
+   *
+   * Should not happen in typical usages, but can occur e.g. if a file reading is aborted. */
+  if (mainvar->next) {
+    BKE_main_free(mainvar->next);
+  }
+
   /* also call when reading a file, erase all, etc */
   ListBase *lbarray[INDEX_ID_MAX];
   int a;