Fix for Java infinite loop encoding into 0-sized buffer (#4654)

java/com/google/flatbuffers/FlatBufferBuilder.java

@@ -196,7 +196,7 @@ public class FlatBufferBuilder {
         int old_buf_size = bb.capacity();
         if ((old_buf_size & 0xC0000000) != 0)  // Ensure we don't grow beyond what fits in an int.
             throw new AssertionError("FlatBuffers: cannot grow buffer beyond 2 gigabytes.");
-        int new_buf_size = old_buf_size << 1;
+        int new_buf_size = old_buf_size == 0 ? 1 : old_buf_size << 1;
         bb.position(0);
         ByteBuffer nbb = bb_factory.newByteBuffer(new_buf_size);
         nbb.position(new_buf_size - old_buf_size);