If ferror is true, we must reset the error indicator.

Submitted by:  Liam J. Foy <liamfoy@sepulcrum.org>
Index: src/usr.bin/rev/rev.c
===================================================================
--- src/usr.bin/rev/rev.c	(revision 117)
+++ src/usr.bin/rev/rev.c	(revision 118)
@@ -103,6 +103,7 @@
 		}
 		if (ferror(fp)) {
 			warn("%s", filename);
+			clearerr(fp);
 			rval = 1;
 		}
 		(void)fclose(fp);