Fix UB in LimitReadBuffer in case of unexpected EOF

Fixes the following possible assertions:
- ./src/IO/ReadBuffer.h:58: bool DB::ReadBuffer::next(): Assertion `!hasPendingData()' failed.
- ./src/IO/LimitReadBuffer.cpp:17: virtual bool DB::LimitReadBuffer::nextImpl(): Assertion `position() >= in->position()' failed.

Fixes: 02151_http_s_structure_set_eof

src/IO/LimitReadBuffer.cpp

@@ -29,7 +29,8 @@ bool LimitReadBuffer::nextImpl()
 
     if (!in->next())
     {
-        working_buffer = in->buffer();
+        /// Clearing the buffer with existing data.
+        set(in->position(), 0);
         return false;
     }