Fix buffer size calculation and memory alignment in evolver_ndf15 | Avoid buffer overflow from input

[kabeleh]

I was investigating a performance and stability issue in my own CLASS fork (kabeleh/iDM), for which I compiled CLASS with the flags

CCFLAG += -fsanitize=undefined 
LDFLAG += -fsanitize=undefined

This reported a memory misalignment issue in evolver_ndf15. There is a (4 bit) int array that is in between doubles (8 bit), which causes a misalignment at the boundary. The fix is relatively simple: move the int array to the end.

This was the TL;DR. In more prose:
The ndf15 evolver allocates a single contiguous buffer and carves it into sub-arrays for doubles, an int array (interpidx), a double-pointer array (dif), and a second double region for backward differences.

The original layout placed the int array (neqp × sizeof(int)) between the double arrays and the double-pointer array:

[15×neqp doubles] [neqp ints] [neqp double*] [(7×neq+1) doubles]

When neqp is odd, neqp × 4 bytes leaves the subsequent double* and double regions at a 4-byte boundary, rather than the 8-byte alignment required for these types. This constitutes undefined behaviour in C11 and is flagged at runtime by the -fsanitize=undefined CCFLAG / LDFLAG as misaligned load/store errors during thermodynamics integration.

The solution is to move the int array to the end of the buffer, after all double and double* regions:

[15×neqp doubles] [neqp double*] [(7×neq+1) doubles] [neqp ints]

Since malloc returns memory aligned to at least 8 bytes, and all regions before the int array are multiples of 8 bytes in size, every double and double* access is now aligned. The int array at the tail has no alignment requirement beyond 4 bytes, so placing it last is safe.

The total buffer size is unchanged.
Verified clean under
-fsanitize=undefined,
-fsanitize=address, and
-fsanitize=thread.

[kabeleh]

The second commit comes at no cost, yet prevents the (unlikely) possibility that line 50 in input.h

    if (flag_temp == _TRUE_)                                                    \
49
    {                                                                           \
50
      strcpy(destination, string_temp);                                         \
51
    }

might cause a buffer overflow. It's very unlikely to encounter this, but no downside in preventing it by increasing the _BASEPATHSIZE_ from 1000 to 1024 in common.h.