Fix some Xcode static analyzer warnings

The only relevant complaint was a memory leak when opening files from
Finder.

src/MacVim/MMAppController.m

@@ -1560,7 +1560,7 @@ fsEventCallback(ConstFSEventStreamRef streamRef,
     if (openFiles != nil)
         *openFiles = dict;
 
-    return files;
+    return [files autorelease];
 }
 
 #if MM_HANDLE_XCODE_MOD_EVENT

src/MacVim/MMPreferenceController.m

@@ -227,7 +227,7 @@ NSString *kOdbEditorIdentifierWriteRoom = @"com.hogbaysoftware.WriteRoom";
     // user changes settings in terminal, the changes are reflected in the
     // dialog)
 
-    NSString *versionString;
+    NSString *versionString = @"";
 
     // Check if ODB path exists before calling isFilePackageAtPath: otherwise
     // an error is output to stderr on Tiger.

src/MacVim/MMTextView.m

@@ -937,7 +937,7 @@
                                    numColumns:ncols];
             [self setNeedsDisplayInRect:invertRects[n]];
         } else {
-            n = numInvertRects = 0;
+            numInvertRects = 0;
         }
     } else {
         // The result should look normal; all we need to do is to mark