fix(transport): initialize body_path to NULL in prepare_http_request

SENTRY_MAKE uses malloc (not calloc), leaving body_path uninitialized.
The curl/winhttp send functions then check `if (req->body_path)` which
passes on garbage, causing a SEGV when dereferencing the pointer.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jpnurmi

src/transports/sentry_http_transport.c

@@ -144,6 +144,7 @@ sentry__prepare_http_request(sentry_envelope_t *envelope,
 
     req->method = "POST";
     req->url = sentry__dsn_get_envelope_url(dsn);
+    req->body_path = NULL;
 
     sentry_prepared_http_header_t *h;
     h = &req->headers[req->headers_len++];