fix: validate cache page index in spiffs_cache_page_free

[Eun0us]

Summary

When a SPIFFS filesystem image is malformed, cp->ix can contain an invalid value (e.g. negative). This leads to undefined behavior in the expression 1 << ix when ix is negative or >= 32 (C standard §6.5.7).

This adds a bounds check at the start of spiffs_cache_page_free() to return SPIFFS_ERR_INTERNAL for out-of-range indices, preventing the UB shift.

How it was found

Found via fuzzing with esp-fuzzer and -fsanitize=undefined:

src/spiffs_cache.c:54:36: runtime error: shift exponent -2 is negative
    #0 spiffs_cache_page_free src/spiffs_cache.c:54:36
    #1 spiffs_cache_drop_page src/spiffs_cache.c:116:5

Test plan

• Verified fix prevents the UBSan error with malformed filesystem image
• Normal cache operations unaffected (valid indices pass the check)