gbox_to_string: Address snprintf warning

[Algunenano]

GCC 7.2 warning in gbox_to_string:

g_box.c: In function 'gbox_to_string'
g_box.c:421:21: warning: '))' directive output may be truncated writing 2 bytes into a region of size between 0 and 105 [-Wformat-truncation=]
   snprintf(str, sz, "GBOX((%.8g,%.8g,%.8g,%.8g),(%.8g,%.8g,%.8g,%.8g))", gbox->xmin, gbox->ymin, gbox->zmin, gbox->mmin, gbox->xmax, gbox->ymax, gbox->zmax, gbox->mmax);
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/stdio.h:862:0,
                 from liblwgeom_internal.h:40,
                 from g_box.c:29:
/usr/include/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' output between 26 and 138 bytes into a destination of size 128
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __bos (__s), __fmt, __va_arg_pack ());
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The destination buffer was set too small to handle the max size of the string.

[Algunenano]

Trac issue: https://trac.osgeo.org/postgis/ticket/3907

[pramsey]

committed