5.2: Don't allow "info args n" to use n outside the valid range

[jansorg]

The max value of n is one less than the current stack depth. info args 0 prints the arguments of stack item ##1.
This PR also does a minor cleanup of args.sh.

Without this fix, an error would be printed:

bashdb<0> info stack
->0 in file `/home/jansorg/test.sh' at line 1
##1 source("/home/jansorg/test.sh") called from file `./bashdb' at line 107
##2 main("-L", ".", "/home/jansorg/test.sh") called from file `./bashdb' at line 0

bashdb<1> info args 2
./command/info_sub/args.sh: Zeile 78: ((: r = r +  : Syntaxfehler: Operator erwartet. (Fehlerverursachendes Zeichen ist "+  ").
Argument count is 0 for this call.

(The error is in German and says "syntax error: expected operator")

[rocky]

LGTM - Thanks!