fix sym_val() buffer overrun

fix sym_val() buffer overrun

Fix 'Bug 3' where too long SYMBOL=string in run-time config file could
overflow a local buffer and clobber the stack.

Valid value is only one character long after processing an 'escaped'
encoded character which can be at most 6 characters (plus terminator):
backslash M backslash and up three digits.  If/when UTF8 gets added
the number of digits will increase.  Use a truncated copy of the input
(substantially bigger than 6+1); ignore any excess.