padding: Avoid calling free on offsets

A 32b compiler has a lot of imagination:

padding.c: In function 'sc_pkcs1_strip_02_padding_constant_time':
padding.c:264:9: error: 'free' called on pointer 'msg_67' with nonzero offset [1, 2147483647] [-Werror=free-nonheap-object]
  264 |         free(msg);
      |         ^~~~~~~~~
padding.c:203:15: note: returned from 'calloc'
  203 |         msg = calloc(n, sizeof(u8));
      |               ^~~~~~~~~~~~~~~~~~~~~

src/libopensc/padding.c

@@ -105,7 +105,7 @@ static int sc_pkcs1_add_01_padding(const u8 *in, size_t in_len,
  memmove(out + i, in, in_len);
  *out++ = 0x00;
  *out++ = 0x01;
- 
+
  memset(out, 0xFF, i - 3);
  out += i - 3;
  *out = 0x00;
@@ -192,15 +192,15 @@ int
 sc_pkcs1_strip_02_padding_constant_time(sc_context_t *ctx, unsigned int n, const u8 *data, unsigned int data_len, u8 *out, unsigned int *out_len)
 {
  unsigned int i = 0;
- u8 *msg = NULL;
+ u8 *msg, *msg_orig = NULL;
  unsigned int good, found_zero_byte, mask;
  unsigned int zero_index = 0, msg_index, mlen = -1, len = 0;
  LOG_FUNC_CALLED(ctx);
 
  if (data == NULL || data_len <= 0 || data_len > n || n < SC_PKCS1_PADDING_MIN_SIZE)
  LOG_FUNC_RETURN(ctx, SC_ERROR_INTERNAL);
 
- msg = calloc(n, sizeof(u8));
+ msg = msg_orig = calloc(n, sizeof(u8));
  if (msg == NULL)
  LOG_FUNC_RETURN(ctx, SC_ERROR_INTERNAL);
 
@@ -261,7 +261,7 @@ sc_pkcs1_strip_02_padding_constant_time(sc_context_t *ctx, unsigned int n, const
  out[i] = constant_time_select_8(mask, msg[msg_index], out[i]);
  }
 
- free(msg);
+ free(msg_orig);
  return constant_time_select(good, mlen, SC_ERROR_WRONG_PADDING);
 }
 
@@ -443,7 +443,7 @@ int sc_pkcs1_strip_digest_info_prefix(unsigned int *algorithm,
  size_t hdr_len = digest_info_prefix[i].hdr_len,
  hash_len = digest_info_prefix[i].hash_len;
  const u8 *hdr = digest_info_prefix[i].hdr;
- 
+
  if (in_len == (hdr_len + hash_len) &&
  !memcmp(in_dat, hdr, hdr_len)) {
  if (algorithm)