SharedArrays: Skip a finally step if we throw before the shmmem_create_pid local is defined (#61141)

In theory, if we throw on line 116 (e.g. if `getpid()` or `randstring()`
throws), then we'll enter the `finally` block before we ever define
`shmmem_create_pid`. I think this case is unlikely, but if we do hit it,
we can't perform the `remotecall_fetch` call in the `finally` block,
because `shmmem_create_pid` won't be defined.

So this PR checks if `shmmem_create_pid` is defined before doing the
`remotecall_fetch` call.

Alternatively, we could lift lines 116 and 117 out of the `try` block,
at which point it will no longer be possible (as far as I can tell) for
us to throw before defining `shmmem_create_pid`.

---

Detected by JET.

Author: DilumAluthge

stdlib/SharedArrays/src/SharedArrays.jl

@@ -153,7 +153,7 @@ function SharedArray{T,N}(dims::Dims{N}; init=false, pids=Int[]) where {T,N}
         shm_seg_name = ""
 
     finally
-        if !isempty(shm_seg_name)
+        if !isempty(shm_seg_name) && @isdefined shmmem_create_pid
             remotecall_fetch(shm_unlink, shmmem_create_pid, shm_seg_name)
         end
     end