hexen: Fix incorrect size on M_StringCopy().

The size parameter for the destination buffer used for this string
copy was one character too short, the result being that patch lump
names using the maximum length (8 characters) were having the last
character cropped off. This in turn caused problems with custom
WADs that added new textures with names like these: the game would
exit on startup with a message like:

  R_InitTextures: Missing patch in texture SKY3GOLD

Amazingly this bug was not noticed because most of the patches in
the Hexen IWAD file have short names of 7 characters or less. The
only exception I noticed was SKYWALL2 which maps to SKYWALL,
another patch, hiding the bug.

Thanks to ETTiNGRiNDER for reporting this bug to me and for
providing me with a private copy of his in-development PWAD that I
could use to find the problem.