lib/utils/pyexec: Forcefully unlock the heap if locked and REPL active.

Otherwise there is really nothing that can be done, it can't be unlocked by
the user because there is no way to allocate memory to execute the unlock.

See issue #4205 and #4209.

docs/library/micropython.rst

@@ -90,6 +90,9 @@ Functions
    in a row and the lock-depth will increase, and then `heap_unlock()` must be
    called the same number of times to make the heap available again.
 
+   If the REPL becomes active with the heap locked then it will be forcefully
+   unlocked.
+
 .. function:: kbd_intr(chr)
 
    Set the character that will raise a `KeyboardInterrupt` exception.  By

lib/utils/pyexec.c

@@ -419,6 +419,12 @@ friendly_repl_reset:
         }
         #endif
 
+        // If the GC is locked at this point there is no way out except a reset,
+        // so force the GC to be unlocked to help the user debug what went wrong.
+        if (MP_STATE_MEM(gc_lock_depth) != 0) {
+            MP_STATE_MEM(gc_lock_depth) = 0;
+        }
+
         vstr_reset(&line);
         int ret = readline(&line, ">>> ");
         mp_parse_input_kind_t parse_input_kind = MP_PARSE_SINGLE_INPUT;