mbedtls_mpi_gcd: fix the case B==0

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/mpi_gcd-0.txt b/ChangeLog.d/mpi_gcd-0.txt
new file mode 100644
index 0000000..41e11e1
--- /dev/null
+++ b/ChangeLog.d/mpi_gcd-0.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix mbedtls_mpi_gcd(G,A,B) when the value of B is zero. This had no
+     effect on Mbed TLS's internal use of mbedtls_mpi_gcd(), but may affect
+     applications that call mbedtls_mpi_gcd() directly. Fixes #4642.