[ssi_stream] Reset m_split_buflen = 0 after handling split buffers

Fixes failure mode where sizeof(uint32_t) + buflen exactly equals the
buffer size, and m_split_buflen had been set to a non-zero value in a
previous iteration.

xroot_ssi_pb/XrdSsiPbIStreamBuffer.hpp

@@ -139,6 +139,8 @@ std::cerr << "[1] " << msg_len << std::endl;
 std::cerr << "[2] " << msg_len << std::endl;
          popRecord(msg_len, split_stream);
       }
+
+      m_split_buflen = 0;
    }
 
    // Extract remaining records from the input buffer