Ensure read_avail is set for the first non-empty block (#6916)
Ensure read_avail is set for the first non-empty block (#6916)

Also add defense to prevent Heap buffer overflow (from ASAN report in prod)
vmamidi/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,SolidWallOfCode/trafficserver,bryancall/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,pbchou/trafficserver,bryancall/trafficserver,SolidWallOfCode/trafficserver,bryancall/trafficserver,duke8253/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,duke8253/trafficserver,duke8253/trafficserver,duke8253/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,bryancall/trafficserver,bryancall/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver