Always use memory management wrappers

Use mem_*() wrappers instead of directly accessing libc functions when
allocating/deallocating memory.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/custom.c

@@ -754,7 +754,7 @@ static void general_option_edit(int i)
 		break;
 	}
 
-	free(buf);
+	mem_free(buf);
 }
 
 /* General configuration. */