Fix incorrect spi active logic, add assert to catch timing issue

Author: multiplemonomials

drivers/source/SPI.cpp

@@ -427,7 +427,7 @@ void SPI::abort_all_transfers()
 
 int SPI::set_dma_usage(DMAUsage usage)
 {
-    if (spi_active(&_peripheral->spi)) {
+    if (_peripheral->initialized && spi_active(&_peripheral->spi)) {
         return -1;
     }
     _usage = usage;

targets/TARGET_NUVOTON/TARGET_M480/spi_api.c

@@ -1004,6 +1004,7 @@ static void spi_dma_handler_tx(uint32_t id, uint32_t event_dma)
     MBED_ASSERT(modinit->modname == (int) obj->spi.spi);
 
     void (*vec)(void) = (void (*)(void)) NVIC_GetVector(modinit->irq_n);
+    MBED_ASSERT(vec != NULL);
     vec();
 }
 
@@ -1027,6 +1028,7 @@ static void spi_dma_handler_rx(uint32_t id, uint32_t event_dma)
     MBED_ASSERT(modinit->modname == (int) obj->spi.spi);
 
     void (*vec)(void) = (void (*)(void)) NVIC_GetVector(modinit->irq_n);
+    MBED_ASSERT(vec != NULL);
     vec();
 }