headers: clear (possibly) lingering pointer in init

The "prevhead" pointer is used for the headers storage but was not
cleared correctly in init, which made it possible to act up when a
handle is reused.

Reported-by: Steve Herrell
Fixes #11101
Closes #11103

lib/headers.c

@@ -336,6 +336,7 @@ CURLcode Curl_headers_push(struct Curl_easy *data, const char *header,
 static void headers_init(struct Curl_easy *data)
 {
   Curl_llist_init(&data->state.httphdrs, NULL);
+  data->state.prevhead = NULL;
 }
 
 /*