Pull Request: Fix memory leak in MjpegClass

[jkarsten]

#Problem
Currently, MjpegClass::setup() allocates _read_buf with malloc(READ_BUFFER_SIZE) every time it is called, but never releases previously allocated memory.
When a MJPEG file is restarted in a loop, this leads to a gradual heap leak (≈1 KB per restart on ESP32). After ~200 restarts the device runs out of memory and crashes.

##Changes

Added cleanup in setup() to free(_read_buf) before allocating a new buffer.

Added a destructor ~MjpegClass() to ensure _read_buf is freed when the object is destroyed.

Changed return value of setup() to (_read_buf != nullptr) so it properly reports allocation failures.

##Modified code (excerpt):

bool setup(
Stream *input, uint8_t *mjpeg_buf, JPEG_DRAW_CALLBACK *pfnDraw, bool useBigEndian,
int x, int y, int widthLimit, int heightLimit)
{
_input = input;
_mjpeg_buf = mjpeg_buf;
_pfnDraw = pfnDraw;
_useBigEndian = useBigEndian;
_x = x;
_y = y;
_widthLimit = widthLimit;
_heightLimit = heightLimit;
_inputindex = 0;

// Prevent memory leak if setup() is called multiple times
if (_read_buf) {
    free(_read_buf);
    _read_buf = nullptr;
}

_read_buf = (uint8_t *)malloc(READ_BUFFER_SIZE);

return (_read_buf != nullptr);

}

// new destructor
~MjpegClass() {
if (_read_buf) {
free(_read_buf);
_read_buf = nullptr;
}
}

##Result

Heap usage on ESP32 remains stable across hundreds of looped video restarts.

No crashes due to gradual memory exhaustion.