tftp warning fix.

At least on Fedora 32 with GCC 10.2.1, dnsmasq compilation emits warning:

tftp.c: In function ‘tftp_request’:
tftp.c:754:3: warning: ‘strcpy’ source argument is the same as
destination [-Wrestrict]
  754 |   strcpy(daemon->namebuff, file);

And indeed it is the same source always on line 477, sometimes also on
571 in tftp.c

Attached patch fixes the warning and possible undefined behaviour on
tftp error.

src/tftp.c

@@ -22,7 +22,7 @@ static void handle_tftp(time_t now, struct tftp_transfer *transfer, ssize_t len)
 static struct tftp_file *check_tftp_fileperm(ssize_t *len, char *prefix);
 static void free_transfer(struct tftp_transfer *transfer);
 static ssize_t tftp_err(int err, char *packet, char *message, char *file);
-static ssize_t tftp_err_oops(char *packet, char *file);
+static ssize_t tftp_err_oops(char *packet, const char *file);
 static ssize_t get_block(char *packet, struct tftp_transfer *transfer);
 static char *next(char **p, char *end);
 static void sanitise(char *buf);
@@ -748,10 +748,11 @@ static ssize_t tftp_err(int err, char *packet, char *message, char *file)
   return  ret;
 }
 
-static ssize_t tftp_err_oops(char *packet, char *file)
+static ssize_t tftp_err_oops(char *packet, const char *file)
 {
   /* May have >1 refs to file, so potentially mangle a copy of the name */
-  strcpy(daemon->namebuff, file);
+  if (file != daemon->namebuff)
+    strcpy(daemon->namebuff, file);
   return tftp_err(ERR_NOTDEF, packet, _("cannot read %s: %s"), daemon->namebuff);
 }