randpkt-test: Fix our optstring

"-a" doesn't have an argument.

Change-Id: Ia178985925da6e8bf9e1f0e9626521fb93656632
Reviewed-on: https://code.wireshark.org/review/18086
Reviewed-by: Gerald Combs <gerald@wireshark.org>

tools/randpkt-test.sh

@@ -18,7 +18,7 @@ ASAN=$CONFIGURED_WITH_ASAN
 WIRESHARK_ABORT_ON_DISSECTOR_BUG="True"
 
 # To do: add options for file names and limits
-while getopts "a:b:d:p:t:" OPTCHAR ; do
+while getopts "ab:d:p:t:" OPTCHAR ; do
     case $OPTCHAR in
         a) ASAN=1 ;;
         b) WIRESHARK_BIN_DIR=$OPTARG ;;