py/objexcept: Make sure mp_obj_new_exception_msg doesn't copy/format msg

mp_obj_new_exception_msg() assumes that the message passed to it is in ROM
and so can use its data directly to create the string object for the
argument of the exception, saving RAM.  At the same time, this approach
also makes sure that there is no attempt to format the message with printf,
which could lead to faults if the message contained % characters.

Fixes issue #3004.

py/objexcept.c

@@ -324,7 +324,35 @@ mp_obj_t mp_obj_new_exception_args(const mp_obj_type_t *exc_type, size_t n_args,
 }
 
 mp_obj_t mp_obj_new_exception_msg(const mp_obj_type_t *exc_type, const char *msg) {
-    return mp_obj_new_exception_msg_varg(exc_type, msg);
+    // Check that the given type is an exception type
+    assert(exc_type->make_new == mp_obj_exception_make_new);
+
+    // Try to allocate memory for the message
+    mp_obj_str_t *o_str = m_new_obj_maybe(mp_obj_str_t);
+
+    #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
+    // If memory allocation failed and there is an emergency buffer then try to use
+    // that buffer to store the string object, reserving room at the start for the
+    // traceback and 1-tuple.
+    if (o_str == NULL
+        && mp_emergency_exception_buf_size >= EMG_BUF_STR_OFFSET + sizeof(mp_obj_str_t)) {
+        o_str = (mp_obj_str_t*)((uint8_t*)MP_STATE_VM(mp_emergency_exception_buf)
+            + EMG_BUF_STR_OFFSET);
+    }
+    #endif
+
+    if (o_str == NULL) {
+        // No memory for the string object so create the exception with no args
+        return mp_obj_exception_make_new(exc_type, 0, 0, NULL);
+    }
+
+    // Create the string object and call mp_obj_exception_make_new to create the exception
+    o_str->base.type = &mp_type_str;
+    o_str->hash = qstr_compute_hash(o_str->data, o_str->len);
+    o_str->len = strlen(msg);
+    o_str->data = (const byte*)msg;
+    mp_obj_t arg = MP_OBJ_FROM_PTR(o_str);
+    return mp_obj_exception_make_new(exc_type, 1, 0, &arg);
 }
 
 // The following struct and function implement a simple printer that conservatively