RT 131375: Fix calculation for maximum array size

We were off by elem_size - log2(elem_size). As we're calculating the
number of bits available for adressing individual array elements, we
have to substract the number of bits for addressing individual bytes in
an array element rather than the number of bytes.
For 64 bit array elements, i.e. 8 bytes, we need 3 bits for addressing
individual bytes. So we lose 3 bits when adressing elements in the
array.

src/6model/reprs/VMArray.c

@@ -1,4 +1,5 @@
 #include "moar.h"
+#include "limits.h"
 
 /* This representation's function pointer table. */
 static const MVMREPROps VMArray_this_repr;
@@ -337,10 +338,24 @@ static void set_size_internal(MVMThreadContext *tc, MVMArrayBody *body, MVMuint6
     else {
         ssize = (n + 0x1000) & ~0xfffUL;
     }
-    if (ssize > (1UL << (8 * sizeof(size_t) - repr_data->elem_size)))
-        MVM_exception_throw_adhoc(tc,
-            "Unable to allocate an array of %"PRIu64" elements",
-            ssize);
+    {
+        /* Our budget is 2^(
+         *     <number of bits in an array index>
+         *     - <number of bits to address individual bytes in an array element>
+         * ) */
+        size_t const elem_addr_size =
+            repr_data->elem_size == 8
+            ? 4
+            : repr_data->elem_size == 4
+                ? 3
+                : repr_data->elem_size == 2
+                    ? 2
+                    : 1;
+        if (ssize > (1UL << (CHAR_BIT * sizeof(size_t) - elem_addr_size)))
+            MVM_exception_throw_adhoc(tc,
+                "Unable to allocate an array of %"PRIu64" elements",
+                ssize);
+    }
 
     /* now allocate the new slot buffer */
     slots = (slots)