(sentence-extractor) Fix resource leak in sentence extractor

The code would always re-initialize the static ngramLexicon and rdrposTagger fields with new instances even if they were already instantiated, leading to a ton of unnecessary RAM allocation.

The modified behavior checks for nullity before creating a new instance.

code/libraries/language-processing/java/nu/marginalia/language/sentence/SentenceExtractor.java

@@ -60,15 +60,18 @@ public class SentenceExtractor {
         }
 
         synchronized (this) {
+            if (ngramLexicon == null) {
                 ngramLexicon = new NgramLexicon(models);
+            }
 
+            if (rdrposTagger == null) {
                 try {
                     rdrposTagger = new RDRPOSTagger(models.posDict, models.posRules);
-            }
-            catch (Exception ex) {
+                } catch (Exception ex) {
                     throw new IllegalStateException(ex);
                 }
             }
+        }
 
     }