Fixed linear_gradient I mode

Author: radarhere

Tests/test_image.py

@@ -521,7 +521,7 @@ def test_linear_gradient(self):
 
         # Arrange
         target_file = "Tests/images/linear_gradient.png"
-        for mode in ["L", "P"]:
+        for mode in ["L", "P", "I"]:
 
             # Act
             im = Image.linear_gradient(mode)

src/libImaging/Fill.c

@@ -76,8 +76,17 @@ ImagingFillLinearGradient(const char *mode) {
         return NULL;
     }
 
-    for (y = 0; y < 256; y++) {
-        memset(im->image8[y], (unsigned char)y, 256);
+    if (im->image8) {
+        for (y = 0; y < 256; y++) {
+            memset(im->image8[y], (unsigned char)y, 256);
+        }
+    } else {
+        int x;
+        for (y = 0; y < 256; y++) {
+            for (x = 0; x < 256; x++) {
+                im->image32[y][x] = y;
+            }
+        }
     }
 
     return im;