bmp/ico decoder: fail early for unsupported bit depths

All the normal bit depths are supported, so no point in trying to go
through the decoding code path for others. Avoids wide bitshift
warning for claimed depths > 32.

Pick-to: 5.15 5.12
Change-Id: I61b72dbbf9558ca28db46f8168339f8174e56997
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/gui/image/qbmphandler.cpp

@@ -262,9 +262,13 @@ static bool read_dib_body(QDataStream &s, const BMP_INFOHDR &bi, qint64 offset,
             depth = 8;
             format = QImage::Format_Indexed8;
             break;
-        default:
+        case 1:
             depth = 1;
             format = QImage::Format_Mono;
+            break;
+        default:
+            return false;
+            break;
     }
 
     if (depth != 32) {

src/plugins/imageformats/ico/qicohandler.cpp

@@ -491,8 +491,12 @@ QImage ICOReader::iconAt(int index)
                 case 4:
                     icoAttrib.depth = 8;
                     break;
-                default:
+                case 1:
                     icoAttrib.depth = 1;
+                    break;
+                default:
+                    return img;
+                    break;
                 }
                 if (icoAttrib.depth == 32)                // there's no colormap
                     icoAttrib.ncolors = 0;