Prevent negative resolution being passed to Locator

[torkleyy]

Fixes #457

Not sure if this is the best way to fix the bug, but it solved the freezing for my application.

[epezent]

1) curious what circumstances got you to a zero width plot 2) probably need to do the same for y-axis locators in the block just above this one

[epezent]

https://github.com/epezent/implot/commit/b5b796a1d4031c445f61b4fb740d5171d8aa56fd