diff: Don't treat null characters like carriage returns in readhash().

The implicit fall-through in the !D_FORCEASCII case caused null
characters to be treated as carriage returns honoring the D_STRIPCR,
D_FOLDBLANKS, and D_IGNOREBLANKS flags.

Reported by:	GCC -Wimplicit-fallthrough
Reviewed by:	bapt
Fixes:	 	3cbf98e2be diff: read whole files to determine if they are ASCII text
Differential Revision:	https://reviews.freebsd.org/D36813

usr.bin/diff/diffreg.c

@@ -1338,6 +1338,7 @@ readhash(FILE *f, int flags, unsigned *hash)
 		case '\0':
 			if ((flags & D_FORCEASCII) == 0)
 				return (RH_BINARY);
+			goto hashchar;
 		case '\r':
 			if (flags & D_STRIPCR) {
 				t = getc(f);
@@ -1356,6 +1357,7 @@ readhash(FILE *f, int flags, unsigned *hash)
 			}
 			/* FALLTHROUGH */
 		default:
+		hashchar:
 			if (space && (flags & D_IGNOREBLANKS) == 0) {
 				i++;
 				space = 0;