oscore_cbor.c: Fix handling large negative integers

Exclude OSS-FUZZ seed corpus files from pre-commit hooks.

.pre-commit-config.yaml

@@ -27,5 +27,7 @@ repos:
     rev: v4.3.0
     hooks:
     -   id: trailing-whitespace
+        exclude: 'tests/oss-fuzz/.*_seed_corpus/.*$'
     -   id: end-of-file-fixer
+        exclude: 'tests/oss-fuzz/.*_seed_corpus/.*$'
     -   id: check-merge-conflict

include/oscore/oscore_cbor.h

@@ -105,7 +105,7 @@ size_t oscore_cbor_put_negative(uint8_t **buffer, size_t *buf_size, int64_t valu
 
 uint8_t oscore_cbor_get_next_element(const uint8_t **buffer, size_t *buf_size);
 
-size_t oscore_cbor_get_element_size(const uint8_t **buffer, size_t *buf_size);
+uint64_t oscore_cbor_get_element_size(const uint8_t **buffer, size_t *buf_size);
 
 uint8_t oscore_cbor_elem_contained(const uint8_t *data, size_t *buf_size,
                                    uint8_t *end);

src/oscore/oscore_cbor.c

@@ -256,10 +256,10 @@ oscore_cbor_get_next_element(const uint8_t **buffer, size_t *buf_len) {
  *   - value of unsigned integer
  */
 
-size_t
+uint64_t
 oscore_cbor_get_element_size(const uint8_t **buffer, size_t *buf_len) {
   uint8_t control = get_byte_inc(buffer, buf_len) & 0x1f;
-  size_t size;
+  uint64_t size;
 
   if (control < 0x18) {
     size = (uint64_t)control;
@@ -289,7 +289,7 @@ oscore_cbor_elem_contained(const uint8_t *data, size_t *buf_len, uint8_t *end) {
 
 int64_t
 oscore_cbor_get_negative_integer(const uint8_t **buffer, size_t *buf_len) {
-  return -(int64_t)(oscore_cbor_get_element_size(buffer, buf_len) + 1);
+  return (int64_t)((uint64_t)(-1) - oscore_cbor_get_element_size(buffer, buf_len));
 }
 
 uint64_t