Initialize ret from test code

The test function mbedtls_mpi_lt_mpi_ct did not initialize ret in test
code. If there was a bug in library code whereby the library function
mbedtls_mpi_lt_mpi_ct() did not set ret when it should, we might have
missed it if ret happened to contain the expected value. So initialize
ret to a value that we never expect.

In Mbed TLS 2.7.17, the lack of initialization also caused Valgrind to
fail on a Clang 3.8 build with -O1 or more (not with -O0). As far as I
can tell, this is an instance of a known bug/feature in Clang which
sometimes generates code that contains a conditional jump based on
memory which is not initialized at the C level. This is not really a
bug in Clang as a C compiler since the code has the same behavior
whether the branch is taken or not, and therefore the branch is not
observable at the C level. However, the branch on C-uninitialized
memory causes a false positive from Valgrind. Here are some reports of
this Clang behavior:
* https://lists.llvm.org/pipermail/llvm-dev/2016-November/107428.html
* https://bugs.llvm.org/show_bug.cgi?id=32604

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/suites/test_suite_mpi.function

@@ -337,7 +337,7 @@ void mbedtls_mpi_lt_mpi_ct( int size_X, char * input_X,
                             int size_Y, char * input_Y,
                             int input_ret, int input_err )
 {
-    unsigned ret;
+    unsigned ret = -1;
     unsigned input_uret = input_ret;
     mbedtls_mpi X, Y;
     mbedtls_mpi_init( &X ); mbedtls_mpi_init( &Y );