Fix EC J-PAKE failing when the payload is all-bits-zero

Fix function mbedtls_ecp_mul_shortcuts() to skip multiplication when m
is 0 and simply assignt 0 to R. Additionally fix ecjpake_zkp_read() to
return MBEDTLS_ERR_ECP_INVALID_KEY when the above condintion is met.

Fix #1792

Signed-off-by: TRodziewicz <rodziewicz@gmail.com>
diff --git a/ChangeLog.d/issue1792.txt b/ChangeLog.d/issue1792.txt
new file mode 100644
index 0000000..e82c80e
--- /dev/null
+++ b/ChangeLog.d/issue1792.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix a bug in EC J-PAKE that would cause it fail when the payload is all-
+     bits-zero.
+     Found by Gilles Peskine, reported in #1792.