Add lock around uv_unref during init (#51236)

This is not a legal operation outside the lock because it's not atomic

Co-authored-by: Valentin Churavy <v.churavy@gmail.com>
(cherry picked from commit bbbcc4f)

base/libuv.jl

@@ -103,8 +103,17 @@ uv_error(prefix::AbstractString, c::Integer) = c < 0 ? throw(_UVError(prefix, c)
 
 eventloop() = ccall(:jl_global_event_loop, Ptr{Cvoid}, ())
 
-uv_unref(h::Ptr{Cvoid}) = ccall(:uv_unref, Cvoid, (Ptr{Cvoid},), h)
-uv_ref(h::Ptr{Cvoid}) = ccall(:uv_ref, Cvoid, (Ptr{Cvoid},), h)
+function uv_unref(h::Ptr{Cvoid})
+    iolock_begin()
+    ccall(:uv_unref, Cvoid, (Ptr{Cvoid},), h)
+    iolock_end()
+end
+
+function uv_ref(h::Ptr{Cvoid})
+    iolock_begin()
+    ccall(:uv_ref, Cvoid, (Ptr{Cvoid},), h)
+    iolock_end()
+end
 
 function process_events()
     return ccall(:jl_process_events, Int32, ())