Simplify thread-safety of mutex initialization.

FossilOrigin-Name: da0587c522163833e5e542f7722aeb69b3183ac1

src/sqliteInt.h

@@ -2938,7 +2938,6 @@ struct Sqlite3Config {
   int szLookaside;                  /* Default lookaside buffer size */
   int nLookaside;                   /* Default lookaside buffer count */
   sqlite3_mem_methods m;            /* Low-level memory allocation interface */
-  sqlite3_mutex_methods *pMutex;    /* Address of mutex member or zero. */
   sqlite3_mutex_methods mutex;      /* Low-level mutex interface */
   sqlite3_pcache_methods2 pcache2;  /* Low-level page-cache interface */
   void *pHeap;                      /* Heap storage space */
@@ -3196,8 +3195,10 @@ const sqlite3_mem_methods *sqlite3MemGetMemsys5(void);
   int sqlite3MutexEnd(void);
 #endif
 #if !defined(SQLITE_MUTEX_OMIT) && !defined(SQLITE_MUTEX_NOOP)
+  void sqlite3MutexCopy(sqlite3_mutex_methods *, sqlite3_mutex_methods const *);
   void sqlite3MemoryBarrier(void);
 #else
+# define sqlite3MutexCopy(x,y)
 # define sqlite3MemoryBarrier()
 #endif
 #if !defined(SQLITE_MUTEX_OMIT)