[BHP1-1622] Initialize total fuel load accumulators to zero

[rjsheperd]

Purpose

SurfaceFuelbedIntermediates::getTotalLiveFuelLoad/getTotalDeadFuelLoad sum onto an uninitialized local totalLoad, which reads as NaN in the Emscripten/WASM build. Initialize to zero.

Related Issues

BHP1-1622

Submission Checklist

• Code compiles are passing (make compile)
• Tests are passing (make test)

Testing

1. Build the WASM bundle in behave-app/behave-lib (make install)
2. Create a SIGSurface, set fuel model 163, and run doSurfaceRun
3. getTotalDeadFuelLoad/getTotalLiveFuelLoad (ton/ac) return 2.15/1.1 instead of NaN