[ARROW-105] Fix BaseAllocator.java NPE when assertions are disabled

When verifying memory using verifyAllocator() method, BaseAllocator throws NPE
if assertions are disabled.

Fixing this issue by checking first if assertion are disabled

Author: laurentgo

java/memory/src/main/java/org/apache/arrow/memory/BaseAllocator.java

@@ -99,6 +99,7 @@ protected BaseAllocator(
 
   }
 
+  @Override
   public void assertOpen() {
     if (AssertionUtil.ASSERT_ENABLED) {
       if (isClosed) {
@@ -287,6 +288,7 @@ public Reservation() {
       }
     }
 
+    @Override
     public boolean add(final int nBytes) {
       assertOpen();
 
@@ -308,6 +310,7 @@ public boolean add(final int nBytes) {
       return true;
     }
 
+    @Override
     public ArrowBuf allocateBuffer() {
       assertOpen();
 
@@ -319,14 +322,17 @@ public ArrowBuf allocateBuffer() {
       return arrowBuf;
     }
 
+    @Override
     public int getSize() {
       return nBytes;
     }
 
+    @Override
     public boolean isUsed() {
       return used;
     }
 
+    @Override
     public boolean isClosed() {
       return closed;
     }
@@ -364,6 +370,7 @@ public void close() {
       closed = true;
     }
 
+    @Override
     public boolean reserve(int nBytes) {
       assertOpen();
 
@@ -509,6 +516,7 @@ public synchronized void close() {
 
   }
 
+  @Override
   public String toString() {
     final Verbosity verbosity = logger.isTraceEnabled() ? Verbosity.LOG_WITH_STACKTRACE
         : Verbosity.BASIC;
@@ -523,6 +531,7 @@ public String toString() {
    *
    * @return A Verbose string of current allocator state.
    */
+  @Override
   public String toVerboseString() {
     final StringBuilder sb = new StringBuilder();
     print(sb, 0, Verbosity.LOG_WITH_STACKTRACE);
@@ -575,13 +584,12 @@ void verifyAllocator() {
    *           when any problems are found
    */
   private void verifyAllocator(final IdentityHashMap<UnsafeDirectLittleEndian, BaseAllocator> buffersSeen) {
-    synchronized (DEBUG_LOCK) {
-
-      // The remaining tests can only be performed if we're in debug mode.
-      if (!DEBUG) {
-        return;
-      }
+    // The remaining tests can only be performed if we're in debug mode.
+    if (!DEBUG) {
+      return;
+    }
 
+    synchronized (DEBUG_LOCK) {
       final long allocated = getAllocatedMemory();
 
       // verify my direct descendants