Use abort() instead of assert(false) for things that should never happen

assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.

This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.

src/nvim/buffer.c

@@ -3270,7 +3270,7 @@ void maketitle(void)
         case 6: buf_p = strappend(buf_p, " -"); break;
         case 5:
         case 7: buf_p = strappend(buf_p, " -+"); break;
-        default: assert(false);
+        default: abort();
       }
 
       if (curbuf->b_fname != NULL) {