Bug#843380: libgvc6 fail to install on stretch

While checking for another bug, I found the cause of this also, libgd3 on my system was corrupted.After reinstalling it installation of libgvc6 has no problem.

Bug#843386: [php-maint] Bug#843386: php7.0-gd: wrong libwebp dependency

> Strange, it doesn't exhibit on my system: > > ondrej@komorebi:~$ ldd /usr/lib/php/20151012/gd.so | grep webp > libwebp.so.6 => /usr/lib/x86_64-linux-gnu/libwebp.so.6 > (0x7f7a085dd000) > ondrej@komorebi:~$ sha256sum /usr/lib/php/20151012/gd.so > 31a113c71efc66abe161f3034cf0c4a89a