smtp: fix compiler warning

- Fix clang string-plus-int warning.

Clang 8 warns about adding a string to an int does not append to the
string. Indeed it doesn't, but that was not the intention either. Use
array indexing as suggested to silence the warning. There should be no
functional changes.

(In other words clang warns about "foo"+2 but not &"foo"[2] so use the
latter.)

smtp.c:1221:29: warning: adding 'int' to a string does not append to the
string [-Wstring-plus-int]
      eob = strdup(SMTP_EOB + 2);
            ~~~~~~~~~~~~~~~~^~~~

Closes https://github.com/curl/curl/pull/3729