LibGfx/ILBMLoader: Don't decode bits once full row has been decoded

We were potentially decoding more bits than needed: this could
trash the next lines if decoder didn't zero the extra bits.

Tests/LibGfx/TestImageDecoder.cpp

@@ -254,6 +254,17 @@ TEST_CASE(test_brush_transparent_color)
     EXPECT_EQ(frame.image->get_pixel(114, 103), Gfx::Color::NamedColor::Black);
 }
 
+TEST_CASE(test_small_24bit)
+{
+    auto file = MUST(Core::MappedFile::map(TEST_INPUT("ilbm/small-24bit.iff"sv)));
+    EXPECT(Gfx::ILBMImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = TRY_OR_FAIL(Gfx::ILBMImageDecoderPlugin::create(file->bytes()));
+
+    auto frame = TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 10, 10 }));
+
+    EXPECT_EQ(frame.image->get_pixel(0, 4), Gfx::Color(1, 0, 1, 255));
+}
+
 TEST_CASE(test_ilbm_malformed_header)
 {
     Array test_inputs = {

Userland/Libraries/LibGfx/ImageFormats/ILBMLoader.cpp

@@ -220,7 +220,9 @@ static ErrorOr<ByteBuffer> planar_to_chunky(ReadonlyBytes bitplanes, ILBMLoading
                 u8 bit = bitplanes[offset_base + i];
                 u8 rgb_shift = p / 8;
 
-                for (u8 b = 0; b < 8; b++) {
+                // Some encoders don't pad bytes rows with 0: make sure we stop
+                // when enough data for current bitplane row has been read
+                for (u8 b = 0; b < 8 && (i * 8) + b < width; b++) {
                     u8 mask = 1 << (7 - b);
                     // get current plane
                     if (bit & mask) {