pack: do not multiply out_size by realloc_size

In `flb_msgpack_raw_to_json_sds`, when the buffer is reallocated it
calls `flb_sds_increase`. This increases the length by adding the `len`
argument to the length and reallocating the sds object. After doing
this, the `out_size` is multiplied by `realloc_size`. This does not
match reality, as what happened in the reallocation was additive not
multiplicative. This commit corrects the inconsistency.

src/flb_pack.c

@@ -810,7 +810,7 @@ flb_sds_t flb_msgpack_raw_to_json_sds(const void *in_buf, size_t in_size)
             tmp_buf = flb_sds_increase(out_buf, realloc_size);
             if (tmp_buf) {
                 out_buf = tmp_buf;
-                out_size *= realloc_size;
+                out_size += realloc_size;
             }
             else {
                 flb_errno();