libsa: exit on EOF in ngets

It was possible in some rare circumstances for ngets to behave terribly with
bhyveload and some form of redirecting user input over a pipe.

PR:		198706
Submitted by:	Ivan Krivonos <int0dster@gmail.com>
MFC after:	1 week

stand/libsa/gets.c

@@ -44,8 +44,11 @@ ngets(char *buf, int n)
     int c;
     char *lp;
 
-    for (lp = buf;;)
-	switch (c = getchar() & 0177) {
+    for (lp = buf;;) {
+	c = getchar();
+	if (c == -1)
+		break;
+	switch (c & 0177) {
 	case '\n':
 	case '\r':
 	    *lp = '\0';
@@ -79,6 +82,7 @@ ngets(char *buf, int n)
 		putchar(c);
 	    }
 	}
+    }
     /*NOTREACHED*/
 }