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).

servers/visual/shader_language.h

@@ -425,6 +425,7 @@ public:
 
 		FunctionNode() {
 			type = TYPE_FUNCTION;
+			return_type = TYPE_VOID;
 			return_precision = PRECISION_DEFAULT;
 			can_discard = false;
 		}