fix: use correct idx into memory for loops

src/main.rs

@@ -22,7 +22,7 @@ fn interpret(code: &str) {
             }
             '[' => {
                 // loop start
-                if memory[pc] == 0 {
+                if memory[head] == 0 {
                     let mut count_brackets = 1;
 
                     // find nested brackets
@@ -47,7 +47,8 @@ fn interpret(code: &str) {
             }
             ']' => {
                 // loop terminator
-                if memory[pc] != 0 {
+                if memory[head] != 0 {
+                    // jump back to loop beginning
                     if let Some(start_pc) = loop_stack.last() {
                         pc = *start_pc;
                     } else {