fix(wear_levelling): guard WL_Flash::write()/read() against size==0 underflow

WL_Flash::write() and WL_Flash::read() computed:

    uint32_t count = (size - 1) / this->cfg.wl_page_size;

`size` is `size_t` (unsigned). Neither the public wl_write()/wl_read() API
(wear_levelling.cpp), nor the newer wl_bdl_write()/wl_bdl_read() block-device
path (wl_blockdev.cpp), reject size == 0 before calling into WL_Flash, and
wear_levelling.h does not document size == 0 as invalid (a 0-byte
write/read is a reasonable no-op, mirroring POSIX write()/read() with
count == 0).

When size == 0, `size - 1` wraps around to SIZE_MAX, so `count` becomes an
enormous page count instead of 0. The functions then loop that many times,
reading (write()) or writing (read()) `wl_page_size` bytes per iteration
through the flash partition, immediately walking past the caller-supplied
buffer on the very first iteration:

  - write(): out-of-bounds *read* from the caller's `src` buffer.
  - read():  out-of-bounds *write* into the caller's `dest` buffer -- the
             more severe case, since it corrupts caller memory with flash
             content instead of merely over-reading.

Verified with a standalone reproduction that compiles the unmodified
WL_Flash.cpp against a mock Flash_Access partition: calling
`wl.write(0, an_8_byte_buffer, 0)` with no other change immediately
segfaults (confirmed count == 0xFFFFFFFF for wl_page_size == 4096); with
this fix applied the same call returns ESP_OK without touching memory
outside the buffer, and normal non-zero-size read/write is unaffected.

Add an early `size == 0` return (mirroring the existing `!initialized`
guard) to both functions, and a host_test regression case exercising
wl_write()/wl_read() with size == 0 through the public API.

Disclosure: this fix was prepared with AI assistance (Claude) and reviewed
by me before submission.

Signed-off-by: yi chen <94xhn1@gmail.com>

components/wear_levelling/host_test/main/test_wl.cpp

@@ -99,6 +99,39 @@ TEST_CASE("write and read back data", "[wear_levelling]")
     free(read);
 }
 
+TEST_CASE("write and read with zero size are safe no-ops", "[wear_levelling]")
+{
+    esp_err_t result;
+    wl_handle_t wl_handle;
+
+    const esp_partition_t *partition = esp_partition_find_first(ESP_PARTITION_TYPE_DATA, ESP_PARTITION_SUBTYPE_ANY, "storage");
+
+    // Mount wear-levelled partition
+    result = wl_mount(partition, &wl_handle);
+    REQUIRE(result == ESP_OK);
+
+    // wl_write()/wl_read() do not document size==0 as invalid (analogous to
+    // POSIX write()/read() with count==0), so it must not be treated as an
+    // out-of-bounds request. Previously, WL_Flash::write()/read() computed
+    // `(size - 1) / wl_page_size` without checking for size==0 first; since
+    // size is unsigned, size==0 wrapped this to a huge page count and walked
+    // far past the caller-provided buffer.
+    uint8_t dummy = 0xAA;
+    result = wl_write(wl_handle, 0, &dummy, 0);
+    REQUIRE(result == ESP_OK);
+
+    uint8_t read_back = 0x55;
+    result = wl_read(wl_handle, 0, &read_back, 0);
+    REQUIRE(result == ESP_OK);
+
+    // Untouched by a genuine zero-length read.
+    REQUIRE(read_back == 0x55);
+
+    // Unmount
+    result = wl_unmount(wl_handle);
+    REQUIRE(result == ESP_OK);
+}
+
 TEST_CASE("power down test", "[wear_levelling]")
 {
     esp_err_t result;