CABI: trap on misaligned error-context.debug-message ptr

[rvolosatovs]

canon_error_context_debug_message stored the debug string's (pointer, length) pair at the guest-supplied ptr with store_string, which writes through store_int without any alignment or bounds check. A misaligned ptr was thus accepted, and an out-of-bounds ptr silently grew the Python bytearray backing linear memory (appending at its current end) instead of trapping.

Check ptr the same way lower_flat_values checks an out-param pointer before storing a spilled tuple: trap unless ptr is aligned to alignment(string) and ptr + elem_size(string) is within memory, then go through store, whose assertions these checks discharge. The checks run before opts.realloc is called.

Refs bytecodealliance/wasmtime#14626