plug-ins: fix #6871 indexed tga file cannot be saved

Exporting an image to TGA fails with a crash when it's an indexed image
with alpha channel. We were not taking into account that even though
the output is 1 byte per pixel, the input should allocate memory for
2 bytes per pixel (1 color index and 1 alpha channel).

(cherry picked from commit 059599fc78)

plug-ins/common/file-tga.c

@@ -1341,7 +1341,10 @@ save_image (const gchar  *filename,
       fputc (0, fp);
     }
 
-  pixels = g_new (guchar, width * out_bpp);
+  if (dtype == GIMP_INDEXEDA_IMAGE)
+    pixels = g_new (guchar, width * 2);
+  else
+    pixels = g_new (guchar, width * out_bpp);
   data   = g_new (guchar, width * out_bpp);
 
   for (row = 0; row < height; ++row)