Fix a memory leak from use of realpath() in HTTP request processing.

[mutability]

realpath() returns a heap-allocated buffer if given NULL for the destination buffer. This must be freed by the caller; dump1090 does not do this.

Instead of worrying about freeing it, take the simpler approach of just providing a stack-allocated destination buffer.

[steel101]

just inserted the code in to my fork it has this and plane count and plane count in title and signal and settings page and lots more including almost all pull request here demo at http://bitropy.net:8080

[mutability]

There's no real need to comment on every pull request you do something with..