The draft n4296 for C11 is explicit that pointing one past an array is perfecly defined: 6.5.6 Language / Expressions / Additive operators:
ยง 8 When an expression that has integer type is added to or subtracted from a pointer, the
result has the type of the pointer operand. … Moreover, if the expression P points to the last
element of an array object, the expression (P)+1 points one past the last element of the
array object, and if the expression Q points one past the last element of an array object,
the expression (Q)-1 points to the last element of the array object… If the result points one past the last element of the array object, it
shall not be used as the operand of a unary * operator that is evaluated.
As the type of the memory is never precised in the sub clause, it applies to any type of memory including allocated one.
That clearly means that after:
int *a = malloc(5 * sizeof(*a));
assert(a != NULL, "Memory allocation failed");
both
int *p = a+5;
int diff = p-a;
are perfectly defined and as the usual pointer arithmetic rules apply, diff shall receive the value 5.