Fix the sqlite3_mutex_alloc() interface to return NULL (not segfault) when

operating in threadsafe mode.  (This is a general-purpose bug fix which
really ought to be ported to trunk.)

FossilOrigin-Name: 64840a3caf569959c9d5e63bdbc0cf675fda3882

src/mutex.c

@@ -77,6 +77,7 @@ int sqlite3MutexEnd(void){
 ** Retrieve a pointer to a static mutex or allocate a new dynamic one.
 */
 sqlite3_mutex *sqlite3_mutex_alloc(int id){
+  if( !sqlite3GlobalConfig.bCoreMutex ) return 0;
 #ifndef SQLITE_OMIT_AUTOINIT
   if( sqlite3_initialize() ) return 0;
 #endif