patch 8.0.1496: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)

src/os_amiga.c

@@ -1619,8 +1619,7 @@ mch_getenv(char_u *var)
     else
 #endif
     {
-	vim_free(alloced);
-	alloced = NULL;
+	VIM_CLEAR(alloced);
 	retval = NULL;
 
 	buf = alloc(IOSIZE);