in stdin: Initialize memory to 0

in_stdin_collect tests !ctx->parser to decide whether a parser
is associated with the context or not.

The problem with that check is ctx->parser is not explictily initialized
in in_stdin_init and the malloc allocation does not guarantee that the
memory assigned to ctx, and ctx->parser is zero initialized. This then
will lead to undefined behavior where sometimes the ctx->parser will not
be 0 and a non existing parser used. Errors like fluent#4544 will then pop up
randomly.

This fix was validated with valgrind and the example provided in fluent#4544

plugins/in_stdin/in_stdin.c

@@ -279,6 +279,7 @@ static int in_stdin_init(struct flb_input_instance *in,
     if (!ctx) {
         return -1;
     }
+    memset(ctx, 0, sizeof(struct flb_in_stdin_config));
 
     /* Initialize stdin config */
     ret = in_stdin_config_init(ctx, in, config);