Avoid warnings compiling seis

[remkos]

Description of proposed changes

This is the squelch warnings when compiling the seis package. @seisman and @joa-quim, please take note!

I have doubts about the lines with the new line numbers 313-319. I did not change the program flow, just avoided the warning about the (second) empty if by moving the semicolon to a new line. But I really wonder if those two ifs are correct, as well as where tensor_end = true; belongs, particularly given the original indentation. Maybe some {} where missing?

Addresses #8653

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[joa-quim]

I have doubts about the lines with the new line numbers 313-319.

Yes, I was staring at them too trying to figure out the meaning (and couldn't). But this is C code that the ISF site provides, so I guess in lack of better information the best is to leave them as they are.