Reject non-positive duration in saveGif to avoid empty-frames crash

[Chessing234]

Bug

saveGif('test', 0) (or any non-positive duration) crashes with TypeError: Cannot read properties of undefined (reading 'length') instead of a friendly error. Reported in #8710.

Root cause

In src/image/loading_displaying.js, saveGif validates that duration is a number but not its range. When duration <= 0:

• nFrames = duration * _frameRate is <= 0
• the while (frameIterator < totalNumberOfFrames) loop never executes
• frames stays []
• _generateGlobalPalette(frames) then reads frames[0].length on line 597 and throws.

Fix

Add a 3-line range check directly after the existing typeof duration check, in the same style as the surrounding validators:

if (duration <= 0) {
  throw new Error('Duration parameter must be a positive number');
}

Why this is correct

• A non-positive duration can never produce a valid GIF, so rejecting it up front is semantically correct and matches the issue reporter's suggested behavior ("saveGif() should reject invalid duration early").
• Preserves the existing validator style (type + range checks at the top of the function).
• No behavioral change for any duration > 0; existing tests in test/unit/image/downloading.js continue to pass unchanged.

Fixes #8710