nil pointer dereference panic when accessing a streaming server that is not implemented

[fernferret]

I believe the invokeServerStream function does not properly check the err on line 265.

This (can) cause a nil panic if the method is not implemented (or another error occurs) on line 267's str.Header().

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Update: Interestingly I'm only seeing this behavior when using github.com/fullstorydev/grpchan. I'm still investigating but I think before a PR is opened the question needs to be asked as to which the proper behavior is. That is to say: How should a Streaming service note that it's not implemented?

This isn't a huge deal as I immediately plan on implementing this service but it was a fun rabbit hole to go down so far, so I'll keep investigating before opening a PR.

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I believe this is as simple as following the pattern used in the invokeUnary function above and am preparing a PR.

Here is an example (trimmed) trace:

invalid memory address or nil pointer dereference goroutine 5 [running]:
panic({0x171fb20, 0x1e92260})
	/usr/local/Cellar/go/1.20.2/libexec/src/runtime/panic.go:884 +0x213
github.com/jhump/protoreflect/dynamic/grpcdynamic.(*ServerStream).Header(...)
	/Users/fernferret/projects/wab/vendor/github.com/jhump/protoreflect/dynamic/grpcdynamic/stub.go:178
github.com/fullstorydev/grpcurl.invokeServerStream({0x1912408, ...
	/Users/fernferret/projects/wab/vendor/github.com/fullstorydev/grpcurl/invoke.go:267 +0x217
github.com/fullstorydev/grpcurl.InvokeRPC({0x1912408, ...
	/Users/fernferret/projects/wab/vendor/github.com/fullstorydev/grpcurl/invoke.go:147 +0x78a
github.com/fullstorydev/grpcui.invokeRPC({0x1912408, ...
	/Users/fernferret/projects/wab/vendor/github.com/fullstorydev/grpcui/handlers.go:464 +0x69c
github.com/fullstorydev/grpcui.RPCInvokeHandlerWithOptions.func1({0x1910cf0, ...
	/Users/fernferret/projects/wab/vendor/github.com/fullstorydev/grpcui/handlers.go:98 +0x3c7
net/http.HandlerFunc.ServeHTTP(0x171abe0?, ...
	/usr/local/Cellar/go/1.20.2/libexec/src/net/http/server.go:2122 +0x2f

Thanks so much for this great library!