Allow for DUORAM_MAXGB to not be set at all

(Assume it's 16 in that case.)

Docker/repro

@@ -50,7 +50,9 @@ fi
 # The maximum amount of memory to use (in GB).  Set the environment
 # variable DUORAM_MAXGB to increase it beyond 16 (don't set it lower
 # than 16).
-if [ "$DUORAM_MAXGB" != "" -a "$DUORAM_MAXGB" -gt 16 ]; then
+if [ "$DUORAM_MAXGB" = "" ]; then
+    maxgb=16
+elif [ "$DUORAM_MAXGB" -gt 16 ]; then
     maxgb=$DUORAM_MAXGB
 else
     maxgb=16