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/charset.c

@@ -1747,7 +1747,7 @@ void vim_str2nr(const char_u *const start, int *const prep, int *const len,
         goto vim_str2nr_dec;
       }
       default: {
-        assert(false);
+        abort();
       }
     }
   } else if ((what & (STR2NR_HEX|STR2NR_OCT|STR2NR_BIN))
@@ -1788,7 +1788,7 @@ void vim_str2nr(const char_u *const start, int *const prep, int *const len,
   }
 
   // Do the string-to-numeric conversion "manually" to avoid sscanf quirks.
-  assert(false);  // Should’ve used goto earlier.
+  abort();  // Should’ve used goto earlier.
 #define PARSE_NUMBER(base, cond, conv) \
   do { \
     while (!STRING_ENDED(ptr) && (cond)) { \