[Bug] SetupAxisLimits causes OOM

ImPlot 0.17 against Dear Imgui 1.92.1 (docking branch)

### Compiler, OS:
clang, Linux 7.0.9

### Details:
`Locator_Default` never terminates on a degenerate axis range when the plot is wide, allocating until OOM.

When an axis range is zero-width (e.g. `SetupAxisLimits(min, max)` with `min == max`, which ImPlotAxis constrains to Max = Min + DBL_EPSILON) and the plot is wide enough that nMajor >= 5 (~1800+ pixels), the computed tick interval `NiceNum(NiceNum(range.Size()*0.99, false)/(nMajor-1), true)` falls below half an ULP of the tick values.

So, `for (double major = graphmin; major < graphmax + 0.5*interval; major += interval)` the addition rounds back to the same double, the loop never advances, and it pushes ticks into the ticker forever.

Observed:
```
range [1.0, 1.0000000000000002]
pixels = 1841
nMajor = 5
interval = 1e-16 vs ULP(1.0) = 2.2e-16
```

Causes 130 M ticks and ~40 GB allocated before the OOM killer intervened).

A minimal fix is to break out of the loop when major + interval == major (or clamp the interval to at least one ULP of the range endpoints).