buffer overflow in gas/app.c

This testcase:
 .irp x x x "
 .end #
 .endr
manages to access lex[EOF].

xxx: Warning: end of file in string; '"' inserted
xxx:1: Warning: missing closing `"'
gas/app.c:844:16: runtime error: index -1 out of bounds for type 'char [256]
Following that there is a buffer overflow.

Stop this happening, and in other similar places, by checking for EOF.