I get the following warning on Debian GNU/Linux when compiling
with -O2.  This is with GCC 4.6.3.

$ CFLAGS='-O2' ./configure && make
...
gdb/cstub.c: In function 'grub_gdb_trap':
gdb/cstub.c:83:21: error: array subscript is above array bounds 
[-Werror=array-bounds]
cc1: all warnings being treated as errors
make[3]: *** [gdb/gdb_module-cstub.o] Error 1
make[3]: Leaving directory `/tmp/GRUB/trunk/grub-core'

Afaics, the out-of-bounds error is real, and the attached patch
fixes it.

Grégoire