[sh] Fix a "may be unused" warning on mips-gcc

mips-gcc for mips32 was complaining that c was potentially used before
being set.  Setting it to 0 before calling fdgetsc() looks like the right
thing to do in this instance; there's an explicit check for c == 0 later
on.

Tested: mips-gcc mips32 build, running /bin/sh on mips32

bin/sh/miscbltin.c

@@ -245,6 +245,7 @@ readcmd(int argc __unused, char **argv __unused)
 	lastnonifs = lastnonifsws = -1;
 	fdctx_init(STDIN_FILENO, &fdctx);
 	for (;;) {
+		c = 0;
 		nread = fdgetc(&fdctx, &c);
 		if (nread == -1) {
 			if (errno == EINTR) {