Fix buffer overflow in CString::AppendFmtV

[B4dM4n]

Description

Fix a (potential) buffer overflow in CString::AppendFmtV.

Compiling nzbget with GCC13 and _FORTIFY_SOURCE can currently lead to the program being terminated:

*** buffer overflow detected ***: terminated

Example backtrace

#0  0x00007fc501ac607c in __pthread_kill_implementation () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#1  0x00007fc501a76e06 in raise () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#2  0x00007fc501a5f8f5 in abort () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#3  0x00007fc501a607a1 in __libc_message.cold () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#4  0x00007fc501b551d9 in __fortify_fail () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#5  0x00007fc501b54b94 in __chk_fail () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#6  0x00007fc501b56929 in __vsnprintf_chk () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#7  0x00000000004e1e3d in CString::AppendFmtV(char const*, __va_list_tag*) ()
#8  0x00000000004e1ef4 in CString::AppendFmt(char const*, ...) ()
#9  0x00000000004a2296 in NzbInfo::BuildFinalDirName() ()
#10 0x000000000048a578 in UnpackController::CreateUnpackDir() ()
#11 0x000000000048cccf in UnpackController::Run() ()
#12 0x00000000004e6092 in Thread::thread_handler() ()
#13 0x00007fc501e13683 in execute_native_thread_routine () from /nix/store/agp6lqznayysqvqkx4k1ggr8n1rsyi8c-gcc-13.2.0-lib/lib/libstdc++.so.6
#14 0x00007fc501ac4383 in start_thread () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6
#15 0x00007fc501b4700c in clone3 () from /nix/store/1rm6sr6ixxzipv5358x0cmaw8rs84g2j-glibc-2.38-44/lib/libc.so.6

It should not actually be possible that a buffer overflow occurs, since the allocated length is checked correctly. But fortify source still detects the wrong length.

Testing

Running with this patch for a few days now and no more crashes so far.

[dnzbk]

Thank you!