The `ed_screen_context()` function is approximately 700 lines long, and
its main structure is a huge chain of `else if` statements. Some of the
bodies did not return, but rather fell through and relied on the `return
-1;` at the bottom of the function. This means that in order to truly
understand what is going on in one of those `else if` blocks, it could
be required to scroll past all the following `else if` blocks,
double-checking that they all had an `else`, and then see what happens
below.
By adding explicit `return -1;` everywhere this happened, this is all
avoided, increasing local understandability of the code. Furthermore, it
makes the upcoming cleanup with the Clang-Tidy rule
`readability-else-after-return` a lot easier to do.
No functional changes.
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