Fix a minor coverity complaint in some apps

Coverity added a new kind of check, and it noticed some minor errors
with some types in two of the apps here, both of the same form:

1. gnu-efi-3.0.6/apps/lfbgrid.c:91: overflow_before_widen: Potentially
overflowing expression "info->VerticalResolution *
info->PixelsPerScanLine" with type "unsigned int" (32 bits, unsigned) is
evaluated using 32-bit arithmetic, and then used in a context that
expects an expression of type "UINTN" (64 bits, unsigned).

1. gnu-efi-3.0.6/apps/bltgrid.c:67: overflow_before_widen: Potentially
overflowing expression "info->VerticalResolution *
info->HorizontalResolution" with type "unsigned int" (32 bits, unsigned)
is evaluated using 32-bit arithmetic, and then used in a context that
expects an expression of type "UINTN" (64 bits, unsigned).

This resolves both issues.

Signed-off-by: Peter Jones <pjones@redhat.com>

apps/bltgrid.c

@@ -64,7 +64,8 @@ draw_boxes(EFI_GRAPHICS_OUTPUT_PROTOCOL *gop)
 		if (CompareMem(info, gop->Mode->Info, sizeof (*info)))
 			continue;
 
-		NumPixels = info->VerticalResolution * info->HorizontalResolution;
+		NumPixels = (UINTN)info->VerticalResolution
+                            * (UINTN)info->HorizontalResolution;
 		BufferSize = NumPixels * sizeof(UINT32);
 
 		PixelBuffer = AllocatePool(BufferSize);

apps/lfbgrid.c

@@ -88,7 +88,8 @@ draw_boxes(EFI_GRAPHICS_OUTPUT_PROTOCOL *gop)
 		if (CompareMem(info, gop->Mode->Info, sizeof (*info)))
 			continue;
 
-		NumPixels = info->VerticalResolution * info->PixelsPerScanLine;
+		NumPixels = (UINTN)info->VerticalResolution
+                            * (UINTN)info->PixelsPerScanLine;
 		BufferSize = NumPixels * sizeof(UINT32);
 		if (BufferSize == gop->Mode->FrameBufferSize) {
 			CopySize = BufferSize;