Fix crash during seeking at the start of the file

Due to seeking optimizations the file can be closed and reopened. The packet's pointer becomes NULL in this case. This change ensures that packet's pointer is valid after seeking performed (if any).

src/FFmpegReader.cpp

@@ -921,6 +921,11 @@ std::shared_ptr<Frame> FFmpegReader::ReadStream(int64_t requested_frame) {
 						continue;
 					}
 
+					// Packet may become NULL on Close inside Seek if CheckSeek returns false
+					if (!packet)
+						// Jump to the next iteration of this loop
+						continue;
+
 					// Get the AVFrame from the current packet
 					frame_finished = GetAVFrame();
 
@@ -957,6 +962,11 @@ std::shared_ptr<Frame> FFmpegReader::ReadStream(int64_t requested_frame) {
 						continue;
 					}
 
+					// Packet may become NULL on Close inside Seek if CheckSeek returns false
+					if (!packet)
+						// Jump to the next iteration of this loop
+						continue;
+
 					// Update PTS / Frame Offset (if any)
 					UpdatePTSOffset(false);