When deleting an SQL function that does not exist, return without doing

anything at all rather than creating a tombstone function.  In this way,
function deletes that happen inside virtual-table destructors that are run
when a database connection is closing do not create new tombstones in the
function table after the function table has already been purged.
[forum:/forumpost/726219164b|forum post 726219164b].

FossilOrigin-Name: 391c73132c80df944fb49a17d8fe78203c54ac48f968ee9dd9dd8c769c0b4b10

src/main.c

@@ -1884,6 +1884,10 @@ int sqlite3CreateFunc(
     }else{
       sqlite3ExpirePreparedStatements(db, 0);
     }
+  }else if( xSFunc==0 && xFinal==0 ){
+    /* Trying to delete a function that does not exist.  This is a no-op.
+    ** https://sqlite.org/forum/forumpost/726219164b */
+    return SQLITE_OK;
   }
 
   p = sqlite3FindFunction(db, zFunctionName, nArg, (u8)enc, 1);