Fix unsigned value representation

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/ExternalImage.kt

@@ -183,8 +183,8 @@ internal operator fun ByteArray.get(range: IntRange): String = buildString {
 }
 
 private fun Byte.fixToString(): String {
-    return when (this.toInt()) {
+    return when (val b = this.toInt() and 0xff) {
         in 0..15 -> "0${this.toString(16).toUpperCase()}"
-        else -> this.toString(16).toUpperCase()
+        else -> b.toString(16).toUpperCase()
     }
 }
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mirai-core/src/commonTest/kotlin/net/mamoe/mirai/utils/ExternalImageTest.kt

@@ -9,6 +9,6 @@ internal class ExternalImageTest {
     fun testByteArrayGet() {
         assertEquals("0F", byteArrayOf(0x0f)[0..0])
         assertEquals("10", byteArrayOf(0x10)[0..0])
-        assertEquals("0FFE", byteArrayOf(0x0F, 0xFE.toByte())[0..0])
+        assertEquals("0FFE", byteArrayOf(0x0F, 0xFE.toByte())[0..1])
     }
 }
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