CXPLAT_DBG_ASSERT(QuicSendCanSendFlagsNow(Send)) during SpinQuic

[nibanks]

Describe the bug

In a recent CI run (on a PR, but unrelated to those changes) SpinQuic hit an assert in the send path:

https://dev.azure.com/ms/msquic/_build/results?buildId=182612&view=logs&j=51112e0e-d4bf-599d-62d4-ebd63a85b53f&t=22852c8a-2900-5132-eae0-719cd7e8f78f&l=14

0b 000000d7`c26fe950 00007ffa`78dc874b msquic!QuicSendFlush+0x517 [D:\a\1\msquic\src\core\send.c @ 1036] 
0c 000000d7`c26ff210 00007ffa`78e062cc msquic!QuicConnDrainOperations+0x67b [D:\a\1\msquic\src\core\connection.c @ 6755] 
0d 000000d7`c26ff470 00007ffa`78e0366e msquic!QuicWorkerProcessConnection+0x59c [D:\a\1\msquic\src\core\worker.c @ 487] 
0e 000000d7`c26ff720 00007ffa`77febf14 msquic!QuicWorkerThread+0x34e [D:\a\1\msquic\src\core\worker.c @ 579] 
0f 000000d7`c26ff950 00007ffa`ab2654e0 clang_rt_asan_dbg_dynamic_x86_64!_asan_wrap_GlobalSize+0x5924a
10 000000d7`c26ff990 00007ffa`abbe05fb kernel32!BaseThreadInitThunk+0x10
11 000000d7`c26ff9c0 00000000`00000000 ntdll!RtlUserThreadStart+0x2b

0:009> dv
             Send = 0x0000125f`5cdcd0d0
StreamPacketCount = 0
           Result = QUIC_SEND_COMPLETE (0n0)
          Builder = struct QUIC_PACKET_BUILDER
             Path = 0x0000125f`5cdcc7b0
    PrevSendFlags = 2
       Connection = 0x0000125f`5cdcc690
           Stream = 0x00000000`00000000
DeadlockDetection = 3
PrevPrevSendFlags = 2

0:009> ?? Send
struct QUIC_SEND * 0x0000125f`5cdcd0d0
   +0x000 FlushOperationPending : 0y1
   +0x000 DelayedAckTimerActive : 0y0
   +0x000 LastFlushTimeValid : 0y1
   +0x000 TailLossProbeNeeded : 0y0
   +0x008 NextPacketNumber : 2
   +0x010 LastFlushTime    : 0x000000a8`82cd2b3d
   +0x018 MaxData          : 0x1000000
   +0x020 PeerMaxData      : 0
   +0x028 OrderedStreamBytesReceived : 0
   +0x030 OrderedStreamBytesSent : 0
   +0x038 SendFlags        : 0
   +0x040 SendStreams      : CXPLAT_LIST_ENTRY
   +0x050 InitialToken     : (null) 
   +0x058 InitialTokenLength : 0

0:009> ?? Builder
struct QUIC_PACKET_BUILDER
   +0x000 Connection       : 0x000000d7`c26feba0 QUIC_CONNECTION
   +0x008 Path             : 0x00005480`2efd3e72 QUIC_PATH
   +0x010 SourceCid        : (null) 
   +0x018 SendData         : 0x00007ffa`78e27796 CXPLAT_SEND_DATA
   +0x020 Datagram         : 0x000000d7`c26fea10 QUIC_BUFFER
   +0x028 Key              : 0x000000d7`c26fef01 QUIC_PACKET_KEY
   +0x030 CipherBatch      : [128]  "???"
   +0x0b0 HpMask           : [128]  ""
   +0x130 HeaderBatch      : [8] (null) 
   +0x170 PacketBatchSent  : 0y0
   +0x170 PacketBatchRetransmittable : 0y0
   +0x170 BatchCount       : 0y0000
   +0x171 TotalCountDatagrams : 0 ''
   +0x172 EncryptionOverhead : 0 ''
   +0x174 EncryptLevel     : 0 ( QUIC_ENCRYPT_LEVEL_INITIAL )
   +0x178 PacketType       : 0 ''
   +0x179 PacketNumberLength : 0 ''
   +0x17a DatagramLength   : 0
   +0x17c TotalDatagramsLength : 0
   +0x180 MinimumDatagramLength : 0
   +0x182 PacketStart      : 0
   +0x184 HeaderLength     : 0
   +0x186 PayloadLengthOffset : 0
   +0x188 SendAllowance    : 0
   +0x190 Metadata         : (null) 
   +0x198 MetadataStorage  : QUIC_MAX_SENT_PACKET_METADATA

Dump File (with symbols): spinquic.exe.zip