aws_util: fix failing sanitizer builds

Signed-off-by: Wesley Pettit <wppttt@amazon.com>

Author: PettitWesley

src/aws/flb_aws_util.c

@@ -720,6 +720,7 @@ flb_sds_t flb_get_s3_key(const char *format, time_t time, const char *tag, char
     flb_sds_t tmp_key = NULL;
     flb_sds_t tmp_tag = NULL;
     struct tm *gmt = NULL;
+    struct flb_sds *head = NULL;
 
     if (strlen(format) > S3_KEY_SIZE){
         flb_warn("[s3_key] Object key length is longer than the 1024 character limit.");
@@ -832,7 +833,8 @@ flb_sds_t flb_get_s3_key(const char *format, time_t time, const char *tag, char
 
     flb_sds_destroy(s3_key);
     s3_key = tmp;
-    FLB_SDS_HEADER(s3_key)->len = strlen(s3_key);
+    head = FLB_SDS_HEADER(s3_key);
+    head->len = strlen(s3_key);
 
     flb_sds_destroy(tmp_tag);
     return s3_key;