output/email: use SCCalloc for OutputJsonEmailCtx

output/email: use SCCalloc for OutputJsonEmailCtx

email_ctx->fields only gets populated when smtp.custom setting is on.
The fn EveEmailLogJSONCustom is called when either
1. smtp.extended setting is on or,
2. email_ctx->fields is populated which means smtp.custom setting is on

In case neither of these are set in suricata.yaml, no call should
ideally be made to the fn EveEmailLogJSONCustom.
However, it turns out that email_ctx->fields is unset and then set only
after the smtp config was found. This leads to email_ctx->fields
sometimes contain value even when no config was given to the smtp
section and can lead to unexpected output.

Fix this by using SCCalloc while initializing OutputJsonEmailCtx struct
instead of SCMalloc.

Bug 6380