Provide and use internal function mbedtls_zeroize_and_free()

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/dhm.c b/library/dhm.c
index 94137a2..174137d 100644
--- a/library/dhm.c
+++ b/library/dhm.c
@@ -617,8 +617,7 @@
     if (fread(*buf, 1, *n, f) != *n) {
         fclose(f);
 
-        mbedtls_platform_zeroize(*buf, *n + 1);
-        mbedtls_free(*buf);
+        mbedtls_zeroize_and_free(*buf, *n + 1);
 
         return MBEDTLS_ERR_DHM_FILE_IO_ERROR;
     }
@@ -649,8 +648,7 @@
 
     ret = mbedtls_dhm_parse_dhm(dhm, buf, n);
 
-    mbedtls_platform_zeroize(buf, n);
-    mbedtls_free(buf);
+    mbedtls_zeroize_and_free(buf, n);
 
     return ret;
 }