form: Fix memory leak

sol_buffer_init is being called inside a loop without the
sol_buffer_fini causing the previous allocated data being lost.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

src/modules/flow/form/form.c

@@ -3226,6 +3226,7 @@ string_formatted_open(struct sol_flow_node *node,
         numeric_field_present = true;
 
         ptr = tmp + 1;
+        sol_buffer_fini(&mdata->formatted_value);
     }
 
     if (!numeric_field_present) {