Fix 'change user password' overriding existing data

www/common/cryptpad-common.js

@@ -1604,6 +1604,26 @@ define([
                 var allocated = Login.allocateBytes(bytes);
                 blockKeys = allocated.blockKeys;
             }));
+        }).nThen(function (waitFor) {
+            var blockUrl = Block.getBlockUrl(blockKeys);
+            // Check whether there is a block at that location
+            Util.fetch(blockUrl, waitFor(function (err, block) {
+                // If there is no block or the block is invalid, continue.
+                if (err) {
+                    console.log("no block found");
+                    return;
+                }
+
+                var decryptedBlock = Block.decrypt(block, blockKeys);
+                if (!decryptedBlock) {
+                    console.error("Found a login block but failed to decrypt");
+                    return;
+                }
+
+                // If there is already a valid block, abort! We risk overriding another user's data
+                waitFor.abort();
+                cb({ error: 'EEXISTS' });
+            }));
         }).nThen(function (waitFor) {
             // Write the new login block
             var temp = {

www/settings/inner.js

@@ -488,6 +488,7 @@ define([
                         UI.removeLoadingScreen();
                         if (obj && obj.error) {
                             // TODO
+                            // XXX EEXISTS error message?
                             UI.alert(Messages.settings_changePasswordError);
                         }
                     });