[core] Fixed the RCV buff nonread position update

condition in case of dropping upto a sequence number.

srtcore/buffer_rcv.cpp

@@ -239,7 +239,7 @@ int CRcvBuffer::dropUpTo(int32_t seqno)
 
     // If the nonread position is now behind the starting position, set it to the starting position and update.
     // Preceding packets were likely missing, and the non read position can probably be moved further now.
-    if (CSeqNo::seqcmp(m_iFirstNonreadPos, m_iStartPos) < 0)
+    if (!isInRange(m_iStartPos, m_iMaxPosOff, m_szSize, m_iFirstNonreadPos))
     {
         m_iFirstNonreadPos = m_iStartPos;
         updateNonreadPos();

srtcore/buffer_rcv.h

@@ -239,9 +239,15 @@ class CRcvBuffer
     inline int incPos(int pos, int inc = 1) const { return (pos + inc) % m_szSize; }
     inline int decPos(int pos) const { return (pos - 1) >= 0 ? (pos - 1) : int(m_szSize - 1); }
     inline int offPos(int pos1, int pos2) const { return (pos2 >= pos1) ? (pos2 - pos1) : int(m_szSize + pos2 - pos1); }
+
+    /// @brief Compares the two positions in the receiver buffer relative to the starting position.
+    /// @param pos2 a position in the receiver buffer.
+    /// @param pos1 a position in the receiver buffer.
+    /// @return a positive value if pos2 is ahead of pos1; a negative value, if pos2 is behind pos1; otherwise returns 0.
     inline int cmpPos(int pos2, int pos1) const
     {
-        // XXX maybe not the best implementation, but this keeps up to the rule
+        // XXX maybe not the best implementation, but this keeps up to the rule.
+        // Maybe use m_iMaxPosOff to ensure a position is not behind the m_iStartPos.
         const int off1 = pos1 >= m_iStartPos ? pos1 - m_iStartPos : pos1 + (int)m_szSize - m_iStartPos;
         const int off2 = pos2 >= m_iStartPos ? pos2 - m_iStartPos : pos2 + (int)m_szSize - m_iStartPos;