Add test-case for signature over zero-length data

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/suites/test_suite_pkcs7.function b/tests/suites/test_suite_pkcs7.function
index 62f9f66..9dce25e 100644
--- a/tests/suites/test_suite_pkcs7.function
+++ b/tests/suites/test_suite_pkcs7.function
@@ -125,7 +125,8 @@
     TEST_ASSERT(file != NULL);
 
     datalen = st.st_size;
-    ASSERT_ALLOC(data, datalen);
+    /* Add 1 so that data is non-NULL for zero length input */
+    ASSERT_ALLOC(data, datalen + 1);
     buflen = fread((void *) data, sizeof(unsigned char), datalen, file);
     TEST_EQUAL(buflen, datalen);