Issue #4461: Safety check in parsenumber (ast.c)

Python/ast.c

@@ -3184,6 +3184,7 @@ parsenumber(struct compiling *c, const char *s)
         int imflag;
 #endif
 
+        assert(s != NULL);
         errno = 0;
         end = s + strlen(s) - 1;
 #ifndef WITHOUT_COMPLEX