Added check for another malloc() that might try to allocate 0 bytes.

src/lib/dns/buffer.h

@@ -323,7 +323,7 @@ public:
     /// \brief Assignment operator
     OutputBuffer& operator =(const OutputBuffer& other) {
         uint8_t* newbuff(static_cast<uint8_t*>(malloc(other.allocated_)));
-        if (newbuff == NULL) {
+        if (newbuff == NULL && other.allocated_ != 0) {
             throw std::bad_alloc();
         }
         free(buffer_);

src/lib/dns/tests/buffer_unittest.cc

@@ -224,6 +224,8 @@ TEST_F(BufferTest, outputBufferZeroSize) {
     EXPECT_NO_THROW({
         OutputBuffer first(0);
         OutputBuffer copy(first);
+        OutputBuffer second(0);
+        second = first;
     });
 }