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/os_mac_conv.c b/src/os_mac_conv.c
index 1fddc61..2bd337e 100644
--- a/src/os_mac_conv.c
+++ b/src/os_mac_conv.c
@@ -480,10 +480,7 @@
 	    if (TECConvertText(gPathConverter, decompPath,
 			decompLen, &decompLen, result,
 			decompLen, &actualLen) != noErr)
-	    {
-		vim_free(result);
-		result = NULL;
-	    }
+		VIM_CLEAR(result);
 	}
     }
 
@@ -517,10 +514,7 @@
 	    utf8_len += inputRead;
 	}
 	else
-	{
-	    vim_free(result);
-	    result = NULL;
-	}
+	    VIM_CLEAR(result);
     }
     else
     {