util/nvmutil: preserve errno during i/o

do not clobber errno

yeah we're basically being libc now

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -1372,6 +1372,7 @@ prw(int fd, void *mem, size_t count,
 {
 	off_t old;
 	ssize_t r;
+	int saved_errno = 0;
 
 	if ((old = lseek_eintr(fd, (off_t)0, SEEK_CUR)) == (off_t)-1)
 		return -1;
@@ -1388,9 +1389,15 @@ prw(int fd, void *mem, size_t count,
 			err(EIO, "%s: Invalid rw_type", path);
 	} while (r < 0 && errno == EINTR);
 
+	if (r < 0)
+		saved_errno = errno;
+
 	if (lseek_eintr(fd, old, SEEK_SET) == (off_t)-1)
 		return -1;
 
+	if (r < 0)
+		errno = saved_errno;
+
 	return r;
 }