aom_image_test: fix implicit conversion warnings

w/clang -fsanitize=integer quiets warnings of the form:
implicit conversion from type 'int' of value -1 (32-bit, signed) to type
'unsigned int' changed the value to 4294967295 (32-bit, unsigned)

Bug: aomedia:3136
Bug: b/229626362
Change-Id: I692e306b623c9d6cb9fcddcb77509d8ce71b8af8
diff --git a/test/aom_image_test.cc b/test/aom_image_test.cc
index 6ee0058..ad48e73 100644
--- a/test/aom_image_test.cc
+++ b/test/aom_image_test.cc
@@ -42,7 +42,9 @@
 
   EXPECT_EQ(aom_img_set_rect(&img, 0, 0, kWidth, kHeight, 0), 0);
   // This would result in overflow because -1 is cast to UINT_MAX.
-  EXPECT_NE(aom_img_set_rect(&img, -1, -1, kWidth, kHeight, 0), 0);
+  EXPECT_NE(aom_img_set_rect(&img, static_cast<unsigned int>(-1),
+                             static_cast<unsigned int>(-1), kWidth, kHeight, 0),
+            0);
 }
 
 TEST(AomImageTest, AomImgAllocNv12) {