whisper : guard null source in buffer loader read callback

[ousamabenyounes]

What

The buffer-based model loader (whisper_init_from_buffer_with_params) installs a
read callback that copies from buf->buffer + current_offset with memcpy.
When the buffer is exhausted — or the supplied buffer is empty/NULL —
size_to_copy is 0 and the source pointer is NULL. Passing a NULL pointer to
memcpy is undefined behavior even for a zero-length copy.

Skip the copy when there is nothing to copy.

This addresses bug 1 of #3879 (the null-pointer-to-memcpy UB). Bug 2 (the
int32 overflow when sizing the mel-filter buffer) is a distinct issue already
covered by the open PR #3780, so it is intentionally left out here.

Why

Loading a crafted/short model through the buffer loader (e.g. via
whisper_init_from_buffer) drives the callback with nothing left to copy.
UBSan flags the memcpy directly.

Test verification (RED → GREEN)

Added tests/test-whisper-buffer-loader.cpp: loading from a NULL/empty buffer
and from a truncated non-model buffer must return NULL without UB. It is
labelled gh so CI's ctest -L gh runs it.

The undefined behavior is deterministic under UBSan (built with
-fsanitize=address,undefined -fno-sanitize-recover=all):

RED (upstream base — memcpy unguarded, new test applied):

/src/src/whisper.cpp:3698:15: runtime error: null pointer passed as argument 2, which is declared to never be null
    #0 ... in operator() /src/src/whisper.cpp:3698
test exit: 1

GREEN (with the fix):

test-whisper-buffer-loader: OK
test exit: 0

Normal-build run (regression guard): the new test plus the existing whisper-cli
tiny test and the UTF-8 unit test all pass.

Refs #3879 (bug 1). Bug 2 is tracked by #3780.