fix: resolve code generation error for interface methods (#775)

jadx-core/src/main/java/jadx/core/codegen/NameGen.java

@@ -139,7 +139,11 @@ public class NameGen {
 	}
 
 	private String getFallbackName(CodeVar var) {
-		return getFallbackName(var.getSsaVars().get(0).getAssign());
+		List<SSAVar> ssaVars = var.getSsaVars();
+		if (ssaVars.isEmpty()) {
+			return "v";
+		}
+		return getFallbackName(ssaVars.get(0).getAssign());
 	}
 
 	private String getFallbackName(RegisterArg arg) {

jadx-core/src/main/java/jadx/core/dex/instructions/args/CodeVar.java

@@ -19,7 +19,9 @@ public class CodeVar {
 		var.setName(mthArg.getName());
 		var.setDeclared(true);
 		var.setThis(mthArg.isThis());
-		var.setSsaVars(Collections.singletonList(new SSAVar(mthArg.getRegNum(), 0, mthArg)));
+		SSAVar ssaVar = new SSAVar(mthArg.getRegNum(), 0, mthArg);
+		ssaVar.setCodeVar(var);
+		var.setSsaVars(Collections.singletonList(ssaVar));
 		return var;
 	}