Fix several Coverity warnings in tftp

Some of the changes are in the libexec/tftpd directory, but to functions that
are only used by tftp(1) (they share some code).

* strcpy => strlcpy (1006793, 1006794, 1006796, 1006741)
* Unchecked return value and TOCTTOU (1009314)
* NULL pointer dereference (1018035, 1018036)

Reported by:	Coverity
CID:		1006793, 1006794, 1006796, 1006741, 1009314, 1018035
CID:		1018036
MFC after:	2 weeks

libexec/tftpd/tftp-io.c

@@ -40,6 +40,7 @@ __FBSDID("$FreeBSD$");
 #include <errno.h>
 #include <setjmp.h>
 #include <signal.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -193,16 +194,16 @@ send_wrq(int peer, char *filename, char *mode)
 
 	tp = (struct tftphdr *)buf;
 	tp->th_opcode = htons((u_short)WRQ);
-	size = 2;
+	size = offsetof(struct tftphdr, th_stuff);
 
 	bp = tp->th_stuff;
-	strcpy(bp, filename);
+	strlcpy(bp, filename, sizeof(buf) - size);
 	bp += strlen(filename);
 	*bp = 0;
 	bp++;
 	size += strlen(filename) + 1;
 
-	strcpy(bp, mode);
+	strlcpy(bp, mode, sizeof(buf) - size);
 	bp += strlen(mode);
 	*bp = 0;
 	bp++;
@@ -241,16 +242,16 @@ send_rrq(int peer, char *filename, char *mode)
 
 	tp = (struct tftphdr *)buf;
 	tp->th_opcode = htons((u_short)RRQ);
-	size = 2;
+	size = offsetof(struct tftphdr, th_stuff);
 
 	bp = tp->th_stuff;
-	strcpy(bp, filename);
+	strlcpy(bp, filename, sizeof(buf) - size);
 	bp += strlen(filename);
 	*bp = 0;
 	bp++;
 	size += strlen(filename) + 1;
 
-	strcpy(bp, mode);
+	strlcpy(bp, mode, sizeof(buf) - size);
 	bp += strlen(mode);
 	*bp = 0;
 	bp++;

libexec/tftpd/tftp-utils.c

@@ -237,14 +237,15 @@ const char *
 debug_show(int d)
 {
 	static char s[100];
+	size_t space = sizeof(s);
 	int i = 0;
 
 	s[0] = '\0';
 	while (debugs[i].name != NULL) {
 		if (d&debugs[i].value) {
-			if (s[0] != '\0') 
-				strcat(s, " ");
-			strcat(s, debugs[i].name);
+			if (s[0] != '\0')
+				strlcat(s, " ", space);
+			strlcat(s, debugs[i].name, space);
 		}
 		i++;
 	}