Fix a few minor issues with SourcePosition.compareTo

sablecc-runtime/src/main/java/de/hhu/stups/sablecc/patch/SourcePosition.java

@@ -25,10 +25,15 @@ public class SourcePosition implements Comparable<SourcePosition>  {
     return pos;
   }
   
+  @Override
   public int compareTo(SourcePosition that) {
-    if (that.line < line) return 1;
-    if (that.line > line) return -1;
-    return pos-that.pos;
+    if (this.line < that.line) {
+      return -1;
+    } else if (this.line > that.line) {
+      return 1;
+    } else {
+      return Integer.compare(this.pos, that.pos);
+    }
   }
   
   @Override