http_client: log allocation failures for request headers

Calls to flb_realloc() normally log allocation failures with
flb_errno(). This commit adds this missing logging.

This should help disambiguate errors in flb_http_do() that are due to
memory issues as opposed to HTTP-level issues.

Signed-off-by: Aaron Jacobs <[email protected]>

src/flb_http_client.c

@@ -1128,6 +1128,7 @@ int flb_http_do(struct flb_http_client *c, size_t *bytes)
         new_size = c->header_size + 2;
         tmp = flb_realloc(c->header_buf, new_size);
         if (!tmp) {
+            flb_errno();
             return -1;
         }
         c->header_buf  = tmp;