internal/transport: Unlock mutex before panic

[fengjun2016]

1. Panic will not actively release the lock.
2. This function is called by a goroutine, if panic occurs, the lock will be held until the entire program exits.

s.hdrMu.Lock()
if p := st.Proto(); p != nil && len(p.Details) > 0 {
	delete(s.trailer, grpcStatusDetailsBinHeader)
	stBytes, err := proto.Marshal(p)
	if err != nil {
		// TODO: return error instead, when callers are able to handle it.
		panic(err)
	}

	h.Set(grpcStatusDetailsBinHeader, encodeBinHeader(stBytes))
}

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.55%. Comparing base (e524655) to head (1ef7635).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7488      +/-   ##
==========================================
+ Coverage   81.40%   81.55%   +0.15%     
==========================================
  Files         357      357              
  Lines       27243    27224      -19     
==========================================
+ Hits        22176    22203      +27     
+ Misses       3850     3813      -37     
+ Partials     1217     1208       -9     

Files	Coverage Δ
internal/transport/handler_server.go	88.00% <100.00%> (-1.34%)	⬇️

... and 22 files with indirect coverage changes