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/eval/encode.c

@@ -174,7 +174,7 @@ static int conv_error(const char *const msg, const MPConvStack *const mpstack,
       case kMPConvPartial: {
         switch (v.data.p.stage) {
           case kMPConvPartialArgs: {
-            assert(false);
+            abort();
             break;
           }
           case kMPConvPartialSelf: {
@@ -237,7 +237,7 @@ bool encode_vim_list_to_buf(const list_T *const list, size_t *const ret_len,
   char *const buf = xmalloc(len);
   size_t read_bytes;
   if (encode_read_from_list(&lrstate, buf, len, &read_bytes) != OK) {
-    assert(false);
+    abort();
   }
   assert(len == read_bytes);
   *ret_buf = buf;