Fix Coverity reports of uninitialized scalar variable

Fixes most current reports on Coverity Scan of uninitialized scalar
variable (CWE-457): https://cwe.mitre.org/data/definitions/457.html

These happen most of the time (in our code) when instanciating structs
without a constructor (or with an incomplete one), and later returning
the instance. This is sometimes intended though, as some parameters are
only used in some situations and should not be double-initialized for
performance reasons (e.g. `constant` in ShaderLanguage::Token).

modules/visual_script/visual_script.cpp

@@ -2028,6 +2028,7 @@ void VisualScriptInstance::create(const Ref<VisualScript> &p_script, Object *p_o
 		function.flow_stack_size = 0;
 		function.pass_stack_size = 0;
 		function.node_count = 0;
+
 		Map<StringName, int> local_var_indices;
 
 		if (function.node < 0) {

modules/visual_script/visual_script.h

@@ -374,12 +374,10 @@ class VisualScriptInstance : public ScriptInstance {
 		int node;
 		int max_stack;
 		int trash_pos;
-		int return_pos;
 		int flow_stack_size;
 		int pass_stack_size;
 		int node_count;
 		int argument_count;
-		bool valid;
 	};
 
 	Map<StringName, Function> functions;