scripts: Validate monitor intervals

Reject zero and nonnumeric sampling intervals before entering the
monitor loops, avoiding division errors and repeated failed sleep
calls.

scripts/monitor-interrupt-rate

@@ -2,6 +2,12 @@
 
 interval="${1:-5}"
 
+if ! [[ "$interval" =~ ^0*[1-9][0-9]*$ ]]; then
+  echo "Error: interval must be a positive integer." >&2
+  exit 1
+fi
+interval="${interval#"${interval%%[!0]*}"}"
+
 echo "Interrupts per second:"
 while true
 do