[bugfix] Fix a livelock issue in MultiLock.Sync.tryAcquireShared(long).

The try method could previously block indefinetly when a thread tried to acquire a mode.
Such an example is t1 acquires IX, and then t2 acquires S; causing t2 to spin indefinetly.
CompatibilityTest surfaced the issue.

Author: adamretter

src/main/java/uk/ac/ic/doc/slurp/multilock/MultiLock.java

@@ -306,6 +306,8 @@ else if (arg == IX_UNIT) {
                                 cachedHoldCounter = rh;
                                 return 1; // still return 1 because IS is always compatible
                             }                            
+                        } else {
+                            return -1;
                         }
                     }
                 }
@@ -332,6 +334,8 @@ else if (arg == S_UNIT) {
                                 cachedHoldCounter = rh;
                                 return 1; // still return 1 because IS is always compatible
                             }                            
+                        } else {
+                            return -1;
                         }
                     }
                 }