Fix compiler warnings in source files (#1704)

Two compiler warnings were addressed:

* In `src/apps/relay/http_server.c`, line 77, a `-Wpointer-sign` warning
occurred when initializing a `char *` with the `uint8_t *` return type
of `ioa_network_buffer_data()`.
* An explicit cast `(char *)` was added to
`ioa_network_buffer_data(nbh_http)` to resolve the type mismatch.

* In `src/apps/relay/acme.c`, line 59, a `-Wchar-subscripts` warning was
present because a `char` variable `c` was used as an array index. `char`
can be signed, potentially leading to negative indices.
* Initially, `c` was cast to `(unsigned char)` at the point of use:
`A[(unsigned char)c]`.
* This was later improved by changing the declaration of `c` from `const
char` to `const unsigned char c = req[k]

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

src/apps/relay/acme.c

@@ -55,7 +55,7 @@ static int is_acme_req(char *req, size_t len) {
     }
     // finally check for allowed chars
     for (size_t k = GET_ACME_PREFIX_LEN; k < i; k++) {
-      const char c = req[k];
+      const unsigned char c = req[k];
       if ((c > 127) || (A[c] == ' ')) {
         return -3;
       }

src/apps/relay/http_server.c

@@ -74,7 +74,7 @@ static void write_http_echo(ioa_socket_handle s) {
                TURN_SOFTWARE, strlen(content_http), content_http);
 
       ioa_network_buffer_handle nbh_http = ioa_network_buffer_allocate(s->e);
-      char *data = ioa_network_buffer_data(nbh_http);
+      char *data = (char *)ioa_network_buffer_data(nbh_http);
 
       strcpy(data, data_http);
       ioa_network_buffer_set_size(nbh_http, strlen(data_http));