read_http_post_inline decodes two hexadecimal digits into one byte at
a time, and it asserts that each hexadecimal digit is valid; however,
both assertions print the first digit when they fail. Fix the assertion
on the second digit to print the second digit.
In uri.post, each file name begins and ends with FILE_CHAR.
Previously, file names were not encoded, and names containing
FILE_CHAR could not be used. Because FILE_CHAR is a control
character, the user cannot directly type it in a file input field,
so ELinks asserted that the field did not contain FILE_CHAR.
However, it is possible to get FILE_CHAR in a file input field
with file name completion (ACT_EDIT_AUTO_COMPLETE), causing the
assertion to fail. Now, ELinks encodes FILE_CHAR as "%02", so it
is no longer ambiguous and the assertion is not needed.
