main.c: Fix double free(3) and refactor form handling

- When a non-empty username and an empty password was given, slcl would
crash due to a double free(3). This happened because append_form would
grow the form list before sanitizing the input and, since the output
pointer was not updated to the caller function, the latter would attempt
to free a now-old pointer.
- Additionally, some compilers such as clang complained about the
potential use of an uninitialized variable when calling forms_free.
- Also, it was a good opportunity to refactor get_forms and its caller
functions, as get_forms was not differentiate fatal errors from user
input errors.

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