util/nvmutil: reset errno on successful file reads

a non-fatal error could have set errno. when we return
from check_read_or_die(), it should be assumed that
all is well.

i don't think this would mask anything important, but
it may be regarded as a preventative bug fix, since
it most likely only prevents false-positives.

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

util/nvmutil/nvmutil.c

@@ -432,8 +432,10 @@ static int
 check_read_or_die(const char *rpath, ssize_t rval, size_t rsize,
     int retry, const char *readtype)
 {
-	if (rval == (ssize_t) rsize)
+	if (rval == (ssize_t) rsize) {
+		errno = 0;
 		return 1; /* Successful read */
+	}
 
 	if (rval != -1)
 		err(ECANCELED, "Short %s, %zd bytes, on file: %s",