test/librbd/fsx.cc: fix BUFFER_SIZE_WARNING

CID 1296375 (yehudasa#1 of 1): Buffer not null terminated (BUFFER_SIZE_WARNING)
 buffer_size_warning: Calling strncpy with a maximum size argument of
 1024 bytes on destination array logfile of size 1024 bytes might leave
 the destination string unterminated.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

src/test/librbd/fsx.cc

@@ -2126,7 +2126,8 @@ main(int argc, char **argv)
 				prt("file name to long\n");
 				exit(1);
 			}
-			strncpy(logfile, dirpath, sizeof(logfile));
+			strncpy(logfile, dirpath, sizeof(logfile)-1);
+			logfile[sizeof(logfile)-1] = '\0';
 			if (strlen(logfile) < sizeof(logfile)-2) {
 				strcat(logfile, "/");
 			} else {