[3.8] Explicitly initialize like the surrounding code (GH-18717)

diff --git a/Modules/_decimal/_decimal.c b/Modules/_decimal/_decimal.c
index 4358c4d..df7c6e2 100644
--- a/Modules/_decimal/_decimal.c
+++ b/Modules/_decimal/_decimal.c
@@ -128,7 +128,7 @@
 /* Invariant: NULL or the most recently accessed thread local context */
 static PyDecContextObject *cached_context = NULL;
 #else
-static PyObject *current_context_var;
+static PyObject *current_context_var = NULL;
 #endif
 
 /* Template for creating new thread contexts, calling Context() without