fix mmap error in OSX

OnDiskInvertedLists.cpp

@@ -233,19 +233,20 @@ void OnDiskInvertedLists::prefetch_lists (const long *list_nos, int n) const
 
 void OnDiskInvertedLists::do_mmap ()
 {
-    const char *rw_flags = read_only ? "r" : "rw+";
+    const char *rw_flags = read_only ? "r" : "r+";
     int prot = read_only ? PROT_READ : PROT_WRITE | PROT_READ;
     FILE *f = fopen (filename.c_str(), rw_flags);
     FAISS_THROW_IF_NOT_FMT (f, "could not open %s in mode %s: %s",
                             filename.c_str(), rw_flags, strerror(errno));
 
-    ptr = (uint8_t*)mmap (nullptr, totsize,
+    uint8_t * ptro = (uint8_t*)mmap (nullptr, totsize,
                           prot, MAP_SHARED, fileno (f), 0);
-
-    FAISS_THROW_IF_NOT_FMT (ptr != MAP_FAILED,
+    
+    FAISS_THROW_IF_NOT_FMT (ptro != MAP_FAILED,
                             "could not mmap %s: %s",
                             filename.c_str(),
                             strerror(errno));
+    ptr = ptro;
     fclose (f);
 
 }
@@ -334,6 +335,8 @@ OnDiskInvertedLists::OnDiskInvertedLists (
 
 OnDiskInvertedLists::OnDiskInvertedLists ():
     InvertedLists (0, 0),
+    totsize (0), 
+    ptr (nullptr), 
     read_only (false),
     locks (new LockLevels ()),
     pf (new OngoingPrefetch (this))