fix(jvm): check for null result from parse callback

Author: ObserverOfTime

ktreesitter/src/androidInstrumentedTest/kotlin/io/github/treesitter/ktreesitter/ParserTest.kt

@@ -69,7 +69,8 @@ class ParserTest : FunSpec({
     test("parse(callback)") {
         val source = "class Foo {}"
         val tree = parser.parse { byte, _ ->
-            source.subSequence(byte.toInt(), minOf(byte.toInt() + 1, source.length))
+            val end = minOf(byte.toInt() * 2, source.length)
+            source.subSequence(byte.toInt(), end).ifEmpty { null }
         }
         tree.text().shouldBeNull()
         tree.rootNode.type shouldBe "program"

ktreesitter/src/commonTest/kotlin/io/github/treesitter/ktreesitter/ParserTest.kt

@@ -66,7 +66,8 @@ class ParserTest : FunSpec({
     test("parse(callback)") {
         val source = "class Foo {}"
         val tree = parser.parse { byte, _ ->
-            source.subSequence(byte.toInt(), minOf(byte.toInt() + 1, source.length))
+            val end = minOf(byte.toInt() * 2, source.length)
+            source.subSequence(byte.toInt(), end).ifEmpty { null }
         }
         tree.text().shouldBeNull()
         tree.rootNode.type shouldBe "program"

ktreesitter/src/jni/parser.c

@@ -57,6 +57,8 @@ static const char *parse_callback(void *payload, uint32_t byte_index, TSPoint po
     (*env)->DeleteLocalRef(env, point);
     if ((*env)->ExceptionCheck(env))
         return NULL;
+    if (char_sequence == NULL)
+        return NULL;
 
     jstring string = (jstring)CALL_METHOD_NO_ARGS(Object, char_sequence, CharSequence_toString);
     (*env)->DeleteLocalRef(env, char_sequence);