plugin: fix leaking invoked method name for exec plugins

plugin: fix leaking invoked method name for exec plugins

The invoked method name was separately duplicated from the call_context
structure. The structure itself is eventually freed by rpc_exec_reply()
but the method string it points to is lost after that.

Use calloc_a() instead to allocate the string copy buffer together with
the context structure, to ensure that all involved memory is freed.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>