Fix: JNI loading of optional SLF4J classes

As mentioned by a Stack Overflow answer: When `FindClass` is
unable to find the class you are looking for, it will throw a
`NoClassDefFoundError`. At the very least, this exception needs
to be cleared before execution proceeds any further. Ref:
https://stackoverflow.com/a/69725342/17173324

Author: dabico

lib/ch_usi_si_seart_treesitter.cc

@@ -394,12 +394,16 @@ jint JNI_OnLoad(JavaVM* vm, void* reserved) {
     "(Lch/usi/si/seart/treesitter/Language;)V")
 
   _loadClass(_loggerClass, "org/slf4j/Logger")
-  if (_loggerClass != NULL) {
+  if (env->ExceptionCheck() == JNI_TRUE) {
+    _clearException();
+  } else {
     _loadMethod(_loggerDebugMethod, _loggerClass, "debug", "(Lorg/slf4j/Marker;Ljava/lang/String;)V");
   }
 
   _loadClass(_markerFactoryClass, "org/slf4j/MarkerFactory")
-  if (_markerFactoryClass != NULL) {
+  if (env->ExceptionCheck() == JNI_TRUE) {
+    _clearException();
+  } else {
     _loadStaticMethod(_markerFactoryGetMarkerStaticMethod, _markerFactoryClass, "getMarker",
       "(Ljava/lang/String;)Lorg/slf4j/Marker;");
   }