Change the expression code generator to account for the fact that the new
sqlite3AtoF() never returns NaN.  Also, clarification of a comment in
where.c.

FossilOrigin-Name: 75f596a04a74eb3a538c7be5b41756c970a21a1b
diff --git a/src/expr.c b/src/expr.c
index 60787fa..0ff234e 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -1708,13 +1708,10 @@
     double value;
     char *zV;
     sqlite3AtoF(z, &value);
-    if( sqlite3IsNaN(value) ){
-      sqlite3VdbeAddOp2(v, OP_Null, 0, iMem);
-    }else{
-      if( negateFlag ) value = -value;
-      zV = dup8bytes(v, (char*)&value);
-      sqlite3VdbeAddOp4(v, OP_Real, 0, iMem, 0, zV, P4_REAL);
-    }
+    assert( !sqlite3IsNaN(value) ); /* The new AtoF never returns NaN */
+    if( negateFlag ) value = -value;
+    zV = dup8bytes(v, (char*)&value);
+    sqlite3VdbeAddOp4(v, OP_Real, 0, iMem, 0, zV, P4_REAL);
   }
 }