linker: do not copy instructions when there is no reference

When resolving the bpf-to-bpf calls we need to copy the
instructions from the program to flatten the references.

But when there is no reference there is nothing to do so we
can skip the copy operation altogether.

linker.go

@@ -207,13 +207,19 @@ func flattenPrograms(progs map[string]*ProgramSpec, names []string) {
 // dependencies of each program.
 func flattenInstructions(name string, progs map[string]*ProgramSpec, refs map[*ProgramSpec][]string) asm.Instructions {
 	prog := progs[name]
+	progRefs := refs[prog]
+
+	if len(progRefs) == 0 {
+		// No references, nothing to do.
+		return prog.Instructions
+	}
 
 	insns := make(asm.Instructions, len(prog.Instructions))
 	copy(insns, prog.Instructions)
 
 	// Add all direct references of prog to the list of to be linked programs.
-	pending := make([]string, len(refs[prog]))
-	copy(pending, refs[prog])
+	pending := make([]string, len(progRefs))
+	copy(pending, progRefs)
 
 	// All references for which we've appended instructions.
 	linked := make(map[string]bool)