Update IXWebSocketHttpHeaders.cpp

[zjp8369]

修改header中值为空时，程序崩溃问题

[bsergean]

Thanks for identifying this bug! I've opened #574 as an alternative fix that addresses the same root cause with a few improvements:

• Adds the missing bounds check on the space-skipping while loop (UB if colon is at the end of the line)
• Replaces the size - start - 2 unsigned arithmetic with a trailing trim — avoids size_t underflow and handles both \r\n and \n-only line endings
• Fixes the isspace UB by casting to unsigned char in the lambda
• Removes the redundant 1024-byte cap (the buffer is already capped at 1023 chars)