Always allocate zt in output of SDL_iconv_string()

Before this, the function could not be used on buffers,
as it would not account for the zero-termination unless
it was included in the input.

(cherry picked from commit 5f5abb6805)

include/SDL_stdinc.h

@@ -688,7 +688,7 @@ extern DECLSPEC size_t SDLCALL SDL_iconv(SDL_iconv_t cd, const char **inbuf,
                                          size_t * outbytesleft);
 
 /**
- * This function converts a string between encodings in one pass, returning a
+ * This function converts a buffer or string between encodings in one pass, returning a
  * string that must be freed with SDL_free() or NULL on error.
  *
  * \since This function is available since SDL 2.0.0.