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>

xen/arch/x86/cpu/mtrr/generic.c

@@ -98,7 +98,7 @@ static const char *__init mtrr_attrib_to_str(mtrr_type x)
 		[MTRR_TYPE_WRBACK]         = "write-back",
 	};
 
-	return x < MTRR_NUM_TYPES ? (strings[x] ?: "?") : "?";
+	return (x < ARRAY_SIZE(strings) && strings[x][0]) ? strings[x] : "?";
 }
 
 static unsigned int __initdata last_fixed_start;

xen/arch/x86/mm/p2m-ept.c

@@ -1204,7 +1204,7 @@ void ept_p2m_uninit(struct p2m_domain *p2m)
 
 static const char *memory_type_to_str(unsigned int x)
 {
-    static const char memory_types[8][2] = {
+    static const char memory_types[8][3] = {
         [MTRR_TYPE_UNCACHABLE]     = "UC",
         [MTRR_TYPE_WRCOMB]         = "WC",
         [MTRR_TYPE_WRTHROUGH]      = "WT",
@@ -1213,7 +1213,8 @@ static const char *memory_type_to_str(unsigned int x)
         [MTRR_NUM_TYPES]           = "??"
     };
 
-    return x < ARRAY_SIZE(memory_types) ? (memory_types[x] ?: "?") : "?";
+    ASSERT(x < ARRAY_SIZE(memory_types));
+    return memory_types[x][0] ? memory_types[x] : "?";
 }
 
 static void ept_dump_p2m_table(unsigned char key)