return 0 on success

tests/gdmatrix/bug00402.c

@@ -17,14 +17,14 @@ int main()
     };
 
     int res = gdAffineInvert(matrix_inv, matrix);
-
+    if (!res) return -1;
     for (int i=0; i < 6; i++) {
         double rounded_res = round(matrix_inv[i] * 10);
         double rounded_exp = round(matrix_inv_exp[i] * 10);
         if (rounded_res != rounded_exp) {
             printf("%i failed %f exp %f", i, matrix_inv[i], matrix_inv_exp[i]);
-            return -1;
+            return -i;
         }
     }
-	return res;
+	return 0;
 }