Fix the byte to positive int issue.

After masking the subnet, we convert it from int to byte and store it back to
bytearray.  Then in the matches() function, we convert the masked addr from int
to byte again, which may be converted to a negative value due to overflow.

Also adding a unit test to cover this case.

core/src/androidTest/java/com/github/shadowsocks/net/SubnetTest.kt

@@ -28,10 +28,18 @@ class SubnetTest {
     }
 
     @Test
-    fun matching() {
+    fun matching1() {
         val matcher = Subnet.fromString("1.10.11.12/25")!!.toImmutable()
         Assert.assertFalse(matcher.matches(parseNumericAddress("1.10.10.12").address))
         Assert.assertTrue(matcher.matches(parseNumericAddress("1.10.11.13").address))
         Assert.assertFalse(matcher.matches(parseNumericAddress("1.10.11.212").address))
     }
+
+    @Test
+    fun matching2() {
+        val matcher = Subnet.fromString("14.208.0.0/12")!!.toImmutable()
+        Assert.assertTrue(matcher.matches(parseNumericAddress("14.215.178.36").address))
+        Assert.assertTrue(matcher.matches(parseNumericAddress("14.215.178.37").address))
+        Assert.assertFalse(matcher.matches(parseNumericAddress("1.10.11.212").address))
+    }
 }

core/src/main/java/com/github/shadowsocks/net/Subnet.kt

@@ -57,6 +57,8 @@ class Subnet(val address: InetAddress, val prefixSize: Int) : Comparable<Subnet>
             }
         }
 
+        fun Byte.toPositiveInt() = toInt() and 0xFF
+
         fun matches(b: Immutable) = matches(b.a)
         fun matches(b: ByteArray): Boolean {
             if (a.size != b.size) return false
@@ -66,7 +68,7 @@ class Subnet(val address: InetAddress, val prefixSize: Int) : Comparable<Subnet>
                 ++i
             }
             val mask = 256 - (1 shl i * 8 + 8 - prefixSize)
-            return i * 8 == prefixSize || a[i].toInt() and mask == b[i].toInt() and mask
+            return i * 8 == prefixSize || a[i].toPositiveInt() == b[i].toPositiveInt() and mask
         }
     }
     fun toImmutable() = Immutable(address.address.also {