Sanity check in HiveSplitGenerator available slot calculation

HiveSplitGenerator calculates the number of available slots from available memory like this:

if (getContext() != null) {
  totalResource = getContext().getTotalAvailableResource().getMemory();
  taskResource = getContext().getVertexTaskResource().getMemory();
  availableSlots = totalResource / taskResource;
}

I had a scenario where the total memory was calculated correctly, but the task memory returned -1. This led to error like these:

tez.HiveSplitGenerator: Number of input splits: 1. -3641 available slots, 1.7 waves. Input format is: org.apache.hadoop.hive.ql.io.HiveInputFormat

Estimated number of tasks: -6189 for bucket 1

java.lang.IllegalArgumentException: Illegal Capacity: -6189

Admittedly, this happened during development, and hopefully will not occur on a properly configured cluster. (Although I'm not sure what the issue was on my setup, possibly XMX set higher than physical memory.)

In any case, it feels like setting availableSlots < 1 will never lead to desired behavior, so in such cases we could emit a warning and correct the value to 1.