Fix #948 ollamaclient: add scanner error checking in stream function for handling context cancellation

[nickisworking]

When using ollamaclient, if the application triggers a context cancel for some reason(i.e. handling user cancellation in case of chatbot), and the cancel signal propagates before making the HTTP call (146 line in ollamaclient.go), an error is returned correctly.

However, if the cancel signal propagates after the HTTP call and response.Body is closed, a nil pointer exception occurs at line 152 of ollamallm.go because the scanner loop is not entered and ollamaclient.ChatResponse is not created.

Therefore, while I considered pre-creating ollamaclient.ChatResponse in the oolamallm package, I decided that propagating the context.Canceled error would be a better approach.

Fixes https://github.com/tmc/langchaingo/issues/948

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[nickisworking]

this would fix this issue #948

[nickisworking]

@tmc can I get a review? 👀

[aria3ppp]

@tmc it is a simple fix in a situation when context deadline triggers and then cause a segmentation fault on resp.Message.Content.