fix: add NULL checks after malloc in drawBmp() (#80)

## Problem
`drawBmp()` allocates two row buffers via `malloc()` but doesn't check
if allocations succeed. On low memory, this causes a crash when the NULL
pointers are dereferenced.

## Fix
Add NULL check after both `malloc()` calls. If either fails, log error
and return early.

Changed `lib/GfxRenderer/GfxRenderer.cpp`.

## Test
- Defensive addition only - no logic changes
- Manual device testing appreciated

lib/GfxRenderer/GfxRenderer.cpp

@@ -136,6 +136,13 @@ void GfxRenderer::drawBitmap(const Bitmap& bitmap, const int x, const int y, con
   auto* outputRow = static_cast<uint8_t*>(malloc(outputRowSize));
   auto* rowBytes = static_cast<uint8_t*>(malloc(bitmap.getRowBytes()));
 
+  if (!outputRow || !rowBytes) {
+    Serial.printf("[%lu] [GFX] !! Failed to allocate BMP row buffers\n", millis());
+    free(outputRow);
+    free(rowBytes);
+    return;
+  }
+
   for (int bmpY = 0; bmpY < bitmap.getHeight(); bmpY++) {
     // The BMP's (0, 0) is the bottom-left corner (if the height is positive, top-left if negative).
     // Screen's (0, 0) is the top-left corner.