Add Fernflower Exception transformer

Fernflower fails to decompile any exception handler with an end_pc
(`to` in Fernflower nomenclature) equal to the length of the code array,
even though this is permitted in the class file spec. This transformer
inserts a NOP at the end of any code array that has such an exception
handler.

Signed-off-by: Major <major@emulate.rs>

deob/src/main/java/dev/openrs2/deob/Deobfuscator.kt

@@ -18,6 +18,7 @@ import dev.openrs2.deob.transform.DummyArgTransformer
 import dev.openrs2.deob.transform.DummyLocalTransformer
 import dev.openrs2.deob.transform.EmptyClassTransformer
 import dev.openrs2.deob.transform.ExceptionTracingTransformer
+import dev.openrs2.deob.transform.FernflowerExceptionTransformer
 import dev.openrs2.deob.transform.FieldOrderTransformer
 import dev.openrs2.deob.transform.FinalTransformer
 import dev.openrs2.deob.transform.InvokeSpecialTransformer
@@ -177,7 +178,8 @@ class Deobfuscator(private val input: Path, private val output: Path) {
             MethodOrderTransformer(),
             VisibilityTransformer(),
             OverrideTransformer(),
-            OriginalPcRestoreTransformer()
+            OriginalPcRestoreTransformer(),
+            FernflowerExceptionTransformer()
         )
     }
 }

deob/src/main/java/dev/openrs2/deob/transform/FernflowerExceptionTransformer.kt

@@ -0,0 +1,36 @@
+package dev.openrs2.deob.transform
+
+import com.github.michaelbull.logging.InlineLogger
+import dev.openrs2.asm.classpath.ClassPath
+import dev.openrs2.asm.classpath.Library
+import dev.openrs2.asm.nextReal
+import dev.openrs2.asm.transform.Transformer
+import org.objectweb.asm.Opcodes
+import org.objectweb.asm.tree.ClassNode
+import org.objectweb.asm.tree.InsnNode
+import org.objectweb.asm.tree.MethodNode
+
+class FernflowerExceptionTransformer : Transformer() {
+    private var nopsInserted = 0
+
+    override fun preTransform(classPath: ClassPath) {
+        nopsInserted = 0
+    }
+
+    override fun transformCode(classPath: ClassPath, library: Library, clazz: ClassNode, method: MethodNode): Boolean {
+        if (method.tryCatchBlocks.any { it.end.nextReal == null }) {
+            method.instructions.add(InsnNode(Opcodes.NOP))
+            nopsInserted++
+        }
+
+        return false
+    }
+
+    override fun postTransform(classPath: ClassPath) {
+        logger.info { "Inserted $nopsInserted NOPs to correct Fernflower's exception generation" }
+    }
+
+    private companion object {
+        private val logger = InlineLogger()
+    }
+}