24062026 Coverity fixes

[rlm2002]

Description

• Logically dead code
  • 561219: remove XBADFILE check
  • 561215/561216: remove dead check
• Untrusted loop bound
  • 561218: Add bounds check to test helper
• Overflowed return value
  • 561217: move casts and harden function
• Unchecked return value
• 561216/561205: Remove call to remove(pubFile/privFile), add (void) cast
• Assign instead of compare
  • 561214: change ExpectTrue() to assignment
• Untrusted pointer read
  • 561207: Bound tainted lengths in ECH test helper

Testing

./configure --enable-all && make check