render: fix drsize() skipping one byte past NUL

This would work if the string has double NUL, which is common enough
that this bug never came up before. x)

src/render/topti.c

@@ -169,7 +169,6 @@ char const *drsize(char const *str_char, font_t const *f, int width, int *w)
 		code_point = topti_utf8_next(&str);
 		if(!code_point)
 		{
-			str_char = (void *)str;
 			break;
 		}