[librsvg/librsvg-2.44] Fix a feConvolveMatrix crash when kernel isn't set

[Federico Mena Quintero <federico src gnome org>]

commit 154de9794f2e639fd661bf25a4c86a3bfece3c0f
Author: Ivan Molodetskikh <yalterz gmail com>
Date:   Thu Sep 13 20:52:31 2018 +0300

    Fix a feConvolveMatrix crash when kernel isn't set

 rsvg_internals/src/filters/convolve_matrix.rs          | 8 ++++++++
 tests/fixtures/crash/feConvolveMatrix-empty-kernel.svg | 7 +++++++
 2 files changed, 15 insertions(+)
---
diff --git a/rsvg_internals/src/filters/convolve_matrix.rs b/rsvg_internals/src/filters/convolve_matrix.rs
index 991c14a9..2db20412 100644
--- a/rsvg_internals/src/filters/convolve_matrix.rs
+++ b/rsvg_internals/src/filters/convolve_matrix.rs
@@ -196,6 +196,14 @@ impl NodeTrait for ConvolveMatrix {
             }));
         }
 
+        // kernel_matrix must have been specified.
+        if self.kernel_matrix.borrow().is_none() {
+            return Err(NodeError::value_error(
+                Attribute::KernelMatrix,
+                "the value must be set",
+            ));
+        }
+
         // Default value for the divisor.
         if self.divisor.get().is_none() {
             self.divisor.set(Some(
diff --git a/tests/fixtures/crash/feConvolveMatrix-empty-kernel.svg 
b/tests/fixtures/crash/feConvolveMatrix-empty-kernel.svg
new file mode 100644
index 00000000..d79ff7ee
--- /dev/null
+++ b/tests/fixtures/crash/feConvolveMatrix-empty-kernel.svg
@@ -0,0 +1,7 @@
+<svg version="1.1" baseProfile="full" id="svg-root"
+  width="100%" height="100%" viewBox="0 0 480 360"
+  xmlns="http://www.w3.org/2000/svg"; xmlns:xlink="http://www.w3.org/1999/xlink";>
+        <filter id="convolve1" filterUnits="objectBoundingBox" x="0%" y="0%" width="100%" height="100%">
+          <feConvolveMatrix />
+        </filter>
+</svg>