Implement CStringWrapper#readBuffer to address Truffle warning

Author: eregon

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/common/CArrayWrappers.java

@@ -307,6 +307,17 @@ float readBufferFloat(ByteOrder byteOrder, long byteOffset,
                         @CachedLibrary("this") InteropLibrary thisLib) throws UnsupportedMessageException, InvalidBufferOffsetException {
             return Double.longBitsToDouble(thisLib.readBufferLong(this, byteOrder, byteOffset));
         }
+
+        @ExportMessage
+        void readBuffer(long byteOffset, byte[] destination, int destinationOffset, int length,
+                        @CachedLibrary("this") InteropLibrary thisLib) throws UnsupportedMessageException, InvalidBufferOffsetException {
+            if (length < 0) {
+                throw InvalidBufferOffsetException.create(byteOffset, length);
+            }
+            for (int i = 0; i < length; i++) {
+                destination[destinationOffset + i] = thisLib.readBufferByte(this, byteOffset + i);
+            }
+        }
     }
 
     /**