Change the definition of SQLITE_CONFIG_SCRATCH so that at most one scratch

buffer is used per thread.  Use the generic heap memory allocator for the
WalIterator object when running a checkpoint.

FossilOrigin-Name: 391c9b85abcb5ba300fb2e116384639310c69ed2

src/malloc.c

@@ -377,11 +377,12 @@ void *sqlite3ScratchMalloc(int n){
 
 
 #if SQLITE_THREADSAFE==0 && !defined(NDEBUG)
-  /* Verify that no more than two scratch allocations per thread
-  ** are outstanding at one time.  (This is only checked in the
-  ** single-threaded case since checking in the multi-threaded case
-  ** would be much more complicated.) */
-  assert( scratchAllocOut<=1 );
+  /* EVIDENCE-OF: R-12970-05880 SQLite will not use more than one scratch
+  ** buffers per thread.
+  **
+  ** This can only be checked in single-threaded mode.
+  */
+  assert( scratchAllocOut==0 );
   if( p ) scratchAllocOut++;
 #endif