Check whether create_tempfile() fails.

Thanks and a tip of the Hatlo hat to Coverity for finding this.

Change-Id: Ie9d4089443e52ef427e0cc8ae6e90a9d9787134e
Reviewed-on: https://code.wireshark.org/review/12123
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wiretap/file_access.c

@@ -2272,6 +2272,11 @@ wtap_dump_open_tempfile_ng(char **filenamep, const char *pfx,
 
 	/* Choose a random name for the file */
 	fd = create_tempfile(&tmpname, pfx);
+	if (fd == -1) {
+		*err = errno;
+		g_free(wdh);
+		return NULL;	/* can't create file */
+	}
 	*filenamep = tmpname;
 
 	/* In case "fopen()" fails but doesn't set "errno", set "errno"