lib/mpi/mpicoder.c: looping issue, need stop when equal to zero, found by 'EXTRA_FLAGS=-W'.

For 'while' looping, need stop when 'nbytes == 0', or will cause issue.
('nbytes' is size_t which is always bigger or equal than zero).

The related warning: (with EXTRA_CFLAGS=-W)

lib/mpi/mpicoder.c:40:2: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

Signed-off-by: Chen Gang <[e-mail]>
Cc: Rusty Russell <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: James Morris <[e-mail]>
Cc: Andy Shevchenko <[e-mail]>
Acked-by: David Howells <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/mpi/mpicoder.c

···
 	mpi_limb_t a;
 	MPI val = NULL;
 
-	while (nbytes >= 0 && buffer[0] == 0) {
+	while (nbytes > 0 && buffer[0] == 0) {
 		buffer++;
 		nbytes--;
 	}