util: Fix buffer overflow in process_field_size_default

[jimmunn]

The process_field_size_default function used a fixed 256-byte buffer for formatting hex output, which overflowed when processing fields larger than 127 bytes (such as Reserved fields up to 302 bytes in Micron SMART logs). This caused segmentation faults.

Additionally, the loop counter was an unsigned char, limiting processing to 255 bytes maximum.

Fix by:

• Converting all process_field_size_* functions to return dynamically allocated strings instead of writing to caller-provided buffers
• Using asprintf() for consistent dynamic allocation
• Using __cleanup_free in the caller for automatic memory management
• Also fixes a pre-existing memory leak where hex_to_ascii() return value was never freed