py/objexcept: Compare mp_emergency_exception_buf_size signed.

mp_emergency_exception_buf_size is signed, so let's make sure we compare
it as such.

py/objexcept.c

@@ -208,7 +208,7 @@ mp_obj_t mp_obj_exception_make_new(const mp_obj_type_t *type, size_t n_args, siz
         // reserved room (after the traceback data) for a tuple with 1 element.
         // Otherwise we are free to use the whole buffer after the traceback data.
         if (o_tuple == NULL && mp_emergency_exception_buf_size >=
-            EMG_BUF_TUPLE_OFFSET + EMG_BUF_TUPLE_SIZE(n_args)) {
+            (mp_int_t)(EMG_BUF_TUPLE_OFFSET + EMG_BUF_TUPLE_SIZE(n_args))) {
             o_tuple = (mp_obj_tuple_t *)
                 ((uint8_t *)MP_STATE_VM(mp_emergency_exception_buf) + EMG_BUF_TUPLE_OFFSET);
         }
@@ -383,7 +383,7 @@ mp_obj_t mp_obj_new_exception_msg(const mp_obj_type_t *exc_type, mp_rom_error_te
     // 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)) {
+        && mp_emergency_exception_buf_size >= (mp_int_t)(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);
     }
@@ -465,7 +465,7 @@ mp_obj_t mp_obj_new_exception_msg_vlist(const mp_obj_type_t *exc_type, mp_rom_er
     // that buffer to store the string object and its data (at least 16 bytes for
     // the string data), reserving room at the start for the traceback and 1-tuple.
     if ((o_str == NULL || o_str_buf == NULL)
-        && mp_emergency_exception_buf_size >= EMG_BUF_STR_OFFSET + sizeof(mp_obj_str_t) + 16) {
+        && mp_emergency_exception_buf_size >= (mp_int_t)(EMG_BUF_STR_OFFSET + sizeof(mp_obj_str_t) + 16)) {
         used_emg_buf = true;
         o_str = (mp_obj_str_t *)((uint8_t *)MP_STATE_VM(mp_emergency_exception_buf) + EMG_BUF_STR_OFFSET);
         o_str_buf = (byte *)((uint8_t *)MP_STATE_VM(mp_emergency_exception_buf) + EMG_BUF_STR_BUF_OFFSET);
@@ -573,7 +573,7 @@ void mp_obj_exception_add_traceback(mp_obj_t self_in, qstr file, size_t line, qs
         self->traceback_data = m_new_maybe(size_t, TRACEBACK_ENTRY_LEN);
         if (self->traceback_data == NULL) {
             #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
-            if (mp_emergency_exception_buf_size >= EMG_BUF_TRACEBACK_OFFSET + EMG_BUF_TRACEBACK_SIZE) {
+            if (mp_emergency_exception_buf_size >= (mp_int_t)(EMG_BUF_TRACEBACK_OFFSET + EMG_BUF_TRACEBACK_SIZE)) {
                 // There is room in the emergency buffer for traceback data
                 size_t *tb = (size_t *)((uint8_t *)MP_STATE_VM(mp_emergency_exception_buf)
                     + EMG_BUF_TRACEBACK_OFFSET);