CVE request - read underflow in libpng 1.2.55, 1.0.65, 1.4.18, and 1.5.25 (pngwutil.c)

[xiaoqixue_1 <xiaoqixue_1 () 163 com>]

there is a underflow read in png_check_keyword in pngwutil.c in libpng-1.2.54, which is found by XiaoQixue and ChenYu.

if the data of "key" is only ' ' (0x20), it will read a byte before the buffer in line 1288.

it also impacts libpng 1.2.55, 1.0.65, 1.4.18, and 1.5.25 .

the details as follows:
https://sourceforge.net/p/libpng/bugs/244/