Use strlen() instead of hard-coding string length

This avoids hard to detect bugs when we want a different string length,
and will be optimised by the compiler anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=711063
Avoid OOB read with buggy servers

If the server doesn't start the Content-Range field with "bytes="
we would have an out-of-bounds read trying to parse the content
of that field. Fall back to a 0 offset when a parsing error occurs.

See https://bugzilla.redhat.com/show_bug.cgi?id=1024020

https://bugzilla.gnome.org/show_bug.cgi?id=711063
Fix clang warning

dmap-md5.c:187:26: warning: 'memset' call operates on objects of type 'MD5_CTX'
while the size is based on a different
      type 'MD5_CTX *' [-Wsizeof-pointer-memaccess]
        memset (ctx, 0, sizeof (ctx));  /* In case it's sensitive */
                ~~~             ^~~

That should be "sizeof(*ctx)" instead.

See https://bugzilla.redhat.com/show_bug.cgi?id=1023528

https://bugzilla.gnome.org/show_bug.cgi?id=711063