iprdbg will fall into an endless loop on stdin EOF

[Low-power]

For example, when pressing ^D on an empty command prompt of iprdbg(8), it will fall into a loop, flooding stdout (usually a terminal) and log file (/var/log/iprdbg):

# iprdbg

ATTENTION: This utility is for adapter developers only! Use at your own risk!

Use "quit" to exit the program.

IPRDB(572A)> Invalid command �. Use "help" for a list of valid commands
IPRDB(572A)> Invalid command �. Use "help" for a list of valid commands
IPRDB(572A)> Invalid command �. Use "help" for a list of valid commands
IPRDB(572A)> Invalid command �. Use "help" for a list of valid commands
IPRDB(572A)> Invalid command �. Use "help" for a list of valid commands
IPRDB(572A)> Invalid command �. Use "help" for a list of valid commands
...

This commit fixes 2 bugs:

• ipr_fgets incorrectly stores the EOF mark ((int)-1) into the output buffer (pointed by buf), resulted in that garble character in above output;
• The main loop in main didn't handle the stdin EOF condition, resulted an endless loop.