BUG/MINOR: cfgparse: Abort parsing the current line if an invalid \x sequence is...

BUG/MINOR: cfgparse: Abort parsing the current line if an invalid \x sequence is encountered

This fixes OSS Fuzz issue https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=21931.

OSS Fuzz detected a hang on configuration parsing for a 200kB line with a large number of
invalid escape sequences. Most likely due to the amounts of error output generated.

This issue is very minor, because usually generated configurations are to be trusted.

The bug exists since at the very least HAProxy 1.4. The patch may be backported if desired.