Remove trail check in the generate_random test

The test function generate_random allocated a few extra bytes after
the expected output and checked that these extra bytes were not
overwritten. Memory sanity checks such as AddressSanitizer and
Valgrind already detect this kind of buffer overflow, so having this
test in our code was actually redundant. Remove it.

This has the benefit of not triggering a build error with GCC
(observed with 7.5.0 and 9.3.0) when ASan+UBSan is enabled: with the
previous code using trail, GCC complained about an excessively large
value passed to calloc(), which was (size_t)(-sizeof(trail)).
Thus this commit fixes #4122.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_psa_crypto.function b/tests/suites/test_suite_psa_crypto.function
index 4cf53ca..ef35d57 100644
--- a/tests/suites/test_suite_psa_crypto.function
+++ b/tests/suites/test_suite_psa_crypto.function
@@ -5543,7 +5543,6 @@
 void generate_random( int bytes_arg )
 {
     size_t bytes = bytes_arg;
-    const unsigned char trail[] = "don't overwrite me";
     unsigned char *output = NULL;
     unsigned char *changed = NULL;
     size_t i;
@@ -5551,9 +5550,8 @@
 
     TEST_ASSERT( bytes_arg >= 0 );
 
-    ASSERT_ALLOC( output, bytes + sizeof( trail ) );
+    ASSERT_ALLOC( output, bytes );
     ASSERT_ALLOC( changed, bytes );
-    memcpy( output + bytes, trail, sizeof( trail ) );
 
     PSA_ASSERT( psa_crypto_init( ) );
 
@@ -5566,10 +5564,6 @@
             memset( output, 0, bytes );
         PSA_ASSERT( psa_generate_random( output, bytes ) );
 
-        /* Check that no more than bytes have been overwritten */
-        ASSERT_COMPARE( output + bytes, sizeof( trail ),
-                        trail, sizeof( trail ) );
-
         for( i = 0; i < bytes; i++ )
         {
             if( output[i] != 0 )