patch 8.2.5004: right shift on negative number does not work as documented

Problem:    Right shift on negative number does not work as documented.
Solution:   Use a uvarnumber_T type cast.

src/charset.c

@@ -2002,6 +2002,7 @@ vim_str2nr(
 	}
 	else
 	{
+	    // prevent a larg unsigned number to become negative
 	    if (un > VARNUM_MAX)
 		un = VARNUM_MAX;
 	    *nptr = (varnumber_T)un;