gdbstub: Fix misuse of isxdigit()

gdbstub: Fix misuse of isxdigit()

gdb_read_byte() passes its @ch argument to isxdigit().  Undefined
behavior when the value is negative.  Two callers:

* gdb_chr_receive() passes an uint8_t value.  Safe.

* gdb_handlesig() a char value.  Unsafe.  Not a security issue,
  because the characters come from the gdb client, which is trusted.

The obvious fix would be casting @ch to unsigned char.  But note that
gdb_read_byte() already casts @ch to uint8_t in many places.  Uses of
@ch without such a cast:

(1) Compare to a character constant with == or !=

(2) s->linesum += ch

(3) Store ch or ch ^ 0x20 into s->line_buf[]

(4) Check for invalid RLE count:
    ch < ' ' || ch == '#' || ch == '$' || ch > 126

(5) Pass to isxdigit()

(6) Pass to fromhex()

Change the parameter type from int to uint8_t, and drop the now
redundant casts.  Affects the above uses as follows:

(1) No change: the character constants are all non-negative.

(2) Effectively no change: we only ever use s->linesum & 0xff, and
    s->linesum is int.

(3) No change: s->line_buf[] is char[].

(4) No change.

(5) Avoid undefined behavior.

(6) No change: only reached when isxdigit(ch)

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190514180311.16028-5-armbru@redhat.com>