openssl: Avoid double semicolons after the GET_BIO_DATA macro

When the macro is expanded with a semicolon following it and the
macro itself contains a semicolon, we ended up in double semicolons,
which is treated as a statement that disallows further declarations.

This avoids errors about mixed declarations and statements on gcc,
after ee050797.

Signed-off-by: Martin Storsjö <martin@martin.st>