cJSON_Utils, cJSON: harden pointer bounds, null checks, and merge patch case sensitivity

[filtede98]

Summary of Changes

This PR provides comprehensive defensive hardening and bug fixes across cJSON_Utils and core cJSON:

1. cJSONUtils_FindPointerFromObjectTo:

   • Check for allocation failure (full_pointer == NULL) in both array and object traversal paths before invoking sprintf or strcat, avoiding NULL pointer dereference (SIGSEGV) on out-of-memory.
   • Verify current_child->string != NULL before computing encoded pointer length and copying.
   • Free target_pointer cleanly upon allocation failures.

2. cJSONUtils_GenerateMergePatchCaseSensitive:

   • Fixed a logic bug where recursive patch generation for nested sub-objects invoked cJSONUtils_GenerateMergePatch(from_child, to_child) (the case-insensitive variant), which caused case sensitivity to be dropped for all nested object members. Now propagates generate_merge_patch(from_child, to_child, case_sensitive).
   • Replaced raw strcmp in generate_merge_patch with compare_strings(..., case_sensitive) to respect case_sensitive and prevent NULL pointer dereferences on missing member names.

3. decode_array_index_from_pointer (RFC 6901):

   • Validated non-NULL pointer and index.
   • Prevented empty array tokens (e.g. "/") from being parsed as index 0 (enforcing position > 0).
   • Added integer overflow detection on parsed_index during decimal accumulation.

4. cJSONUtils_strdup, pointer_encoded_length, and encode_string_as_pointer:

   • Added defensive NULL pointer validation matching cJSON_strdup.

5. insert_item_in_array & detach_item_from_array:

   • Guarded against NULL array and NULL newitem to avoid unconditional dereference of array->child.

6. compare_json:

   • Added NULL checks on valuestring for cJSON_String and cJSON_Raw, avoiding strcmp(NULL, ...) crashes.

7. create_patches:

   • Added allocation failure checks for new_path before formatting with sprintf.
   • Guarded NULL valuestrings in string diffing.

8. cJSON_ReplaceItemViaPointer:

   • Added check (item != parent->child && item->prev == NULL) to prevent list corruption when attempting to replace an unlinked item, consistent with cJSON_DetachItemViaPointer.

9. cJSON_PrintPreallocated:

   • Validated length <= 0, safely returning false for empty/zero-capacity buffers.

10. cJSON_SetValuestring:

    • Replaced strcpy and relational pointer comparison between distinct allocations (valuestring + v1_len < object->valuestring, which constitutes undefined behavior in ISO C) with memmove.

11. Unit Tests:

    • Added unit test coverage in tests/misc_utils_tests.c covering merge patch case sensitivity across nested objects, RFC 6901 pointer array index bounds/overflow, zero-length preallocated print buffer rejection, and unlinked replacement rejection.