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)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 4b34538..f09be92 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -9222,10 +9222,7 @@
 	    if (*q != NUL)
 		STRMOVE(remain, q - 1);
 	    else
-	    {
-		vim_free(remain);
-		remain = NULL;
-	    }
+		VIM_CLEAR(remain);
 	}
 
 	/* If the result is a relative path name, make it explicitly relative to