Only close file if already open

Ensures that the tablebase file is only closed if already open.

Fixes #4268
Closes https://github.com/official-stockfish/Stockfish/pull/4321

No functional change

AUTHORS

@@ -92,6 +92,7 @@ Ivan Ivec (IIvec)
 Jacques B. (Timshel)
 Jan Ondruš (hxim)
 Jared Kish (Kurtbusch, kurt22i)
+Jake Senne (w1wwwwww)
 Jarrod Torriero (DU-jdto)
 Jean Gauthier (OuaisBla)
 Jean-Francois Romang (jromang)

src/syzygy/tbprobe.cpp

@@ -199,13 +199,10 @@ public:
         }
     }
 
-    // Memory map the file and check it. File should be already open and will be
-    // closed after mapping.
+    // Memory map the file and check it.
     uint8_t* map(void** baseAddress, uint64_t* mapping, TBType type) {
-
-        assert(is_open());
-
-        close(); // Need to re-open to get native file descriptor
+        if (is_open())
+            close(); // Need to re-open to get native file descriptor
 
 #ifndef _WIN32
         struct stat statbuf;