PVS-Studio warning V512 for FlightGear

void MD5Final(uint8_t digest[16], struct MD5Context *ctx)
{
  ....
  memset(ctx, 0, sizeof(ctx));  /* In case it's sensitive */
  ....
}

This suspicious code was found in FlightGear project by PVS-Studio static code analyzer.
Warning message is:
V512 A call of the 'memset' function will lead to underflow of the buffer 'ctx'. md5.c 180

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/