Merge pull request #102 from BeatButton/write-image-blob-null

don't segfault if MagickGetImageBlob returns null

src/wand/magick.rs

@@ -831,6 +831,9 @@ impl MagickWand {
             bindings::MagickSetImageFormat(self.wand, c_format.as_ptr());
             bindings::MagickGetImageBlob(self.wand, &mut length)
         };
+        if blob.is_null() {
+            Err(MagickError("failed to write image blob"))
+        } else {
             let mut bytes = Vec::with_capacity(length as usize);
             unsafe {
                 bytes.set_len(length as usize);
@@ -839,6 +842,7 @@ impl MagickWand {
             };
             Ok(bytes)
         }
+    }
 
     /// Write the images in the desired format to a new blob.
     ///