x86: fix errors arising from c/s dab76ff

x86: fix errors arising from c/s dab76ff

Coverity correctly identifies that the changes in mtrr_attrib_to_str()
introduce dead code.  strings[] is a 2d array, rather than an array of
strings, which means that strings[x] will never be a NULL pointer.

Adjust the check to compenstate, by looking for a NUL in strings[x][0]
instead.

Curiously, Coverity did not notice the same error with memory_type_to_str().
There was also a further error; the strings were not NULL terminated, which
made the return type of memory_type_to_str() erronious.

Bump the 2D array to 3 characters, so the strings retain their NUL characters,
and introduce an ASSERT() as requested on one thread of the original patch.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>