In the sorter, only use large memory allocations if scratch memory has not

been configured.  Add #ifdefs to disable unused code when 
SQLITE_MAX_WORKER_THREADS is zero.  Other sorter changes in support
of testability.

FossilOrigin-Name: d7e2b0d9cb099eda3341bc934bedff9facfe88bd

src/main.c

@@ -516,9 +516,11 @@ int sqlite3_config(int op, ...){
 #endif
 
     case SQLITE_CONFIG_WORKER_THREADS: {
+#if SQLITE_MAX_WORKER_THREADS>0
       int n = va_arg(ap, int);
       if( n>SQLITE_MAX_WORKER_THREADS ) n = SQLITE_MAX_WORKER_THREADS;
       if( n>=0 ) sqlite3GlobalConfig.nWorker = n;
+#endif
       break;
     }