ByteBuf#ensureWritable(int, boolean) should not throw

Motivation:
The javadocs for ByteBuf#ensureWritable(int, boolean) indicate that it should not throw, and instead the return code should indicate the result of the operation. Due to a bug in AbstractByteBuf it is possible for a resize to be attempted on a buffer that may exceed maxCapacity() and therefore throw.

Modifications:
- If there is not enough space in the buffer, and force is false, then a resize should not be attempted

Result:
AbstractByteBuf#ensureWritable(int, boolean) enforces the javadoc constraints and does not throw.

buffer/src/main/java/io/netty/buffer/AbstractByteBuf.java

@@ -295,16 +295,16 @@ public abstract class AbstractByteBuf extends ByteBuf {
             return 0;
         }
 
+        final int maxCapacity = maxCapacity();
+        final int writerIndex = writerIndex();
         if (minWritableBytes > maxCapacity - writerIndex) {
-            if (force) {
-                if (capacity() == maxCapacity()) {
+            if (!force || capacity() == maxCapacity) {
                 return 1;
             }
 
-                capacity(maxCapacity());
+            capacity(maxCapacity);
             return 3;
         }
-        }
 
         // Normalize the current capacity to the power of 2.
         int newCapacity = alloc().calculateNewCapacity(writerIndex + minWritableBytes, maxCapacity);

buffer/src/test/java/io/netty/buffer/AbstractByteBufTest.java

@@ -2471,6 +2471,23 @@ public abstract class AbstractByteBufTest {
         buffer.release();
     }
 
+    @Test
+    public void ensureWritableWithForceDoesNotThrow() {
+        ensureWritableDoesNotThrow(true);
+    }
+
+    @Test
+    public void ensureWritableWithOutForceDoesNotThrow() {
+        ensureWritableDoesNotThrow(false);
+    }
+
+    private void ensureWritableDoesNotThrow(boolean force) {
+        final ByteBuf buffer = newBuffer(8);
+        buffer.writerIndex(buffer.capacity());
+        buffer.ensureWritable(8, force);
+        buffer.release();
+    }
+
     // See:
     // - https://github.com/netty/netty/issues/2587
     // - https://github.com/netty/netty/issues/2580