rmtfs: fix segfault

shadow_len is size_t and therefore cannot be negative. Cast both
parameters to ssize_t (which is signed) to correctly use the following
"if (n > 0)"
diff --git a/qcom/rmtfs/storage.c b/qcom/rmtfs/storage.c
index aaf73d0..107b296 100644
--- a/qcom/rmtfs/storage.c
+++ b/qcom/rmtfs/storage.c
@@ -202,7 +202,7 @@
 	if (!storage_read_only) {
 		n = pread(rmtfd->fd, buf, nbyte, offset);
 	} else {
-		n = MIN(nbyte, rmtfd->shadow_len - offset);
+		n = MIN((ssize_t)nbyte, (ssize_t)rmtfd->shadow_len - offset);
 		if (n > 0)
 			memcpy(buf, (char*)rmtfd->shadow_buf + offset, n);
 		else