Fix disassembly INVOKEVIRTUAL without symbols

If there were no debug symbols, the constant pool isn't extended to hold
all methods. This caused a method name resolution to fail if main called
a method. This change takes care of this edge case and immediately
resolves to None, causing the default placeholders to be generated.

src/disassembler.rs

@@ -54,6 +54,9 @@ impl DebugSymbols {
     }
 
     fn lookup_method_idx(&self, idx: usize) -> Option<String> {
+        if idx >= self.constants.len() {
+            return None;
+        }
         match self.methods.get(&self.constants[idx]) {
             Some(name) => Some(name.to_string()),
             None => None,