Fix warning with gcc7/maximum allocation exceeded

Adds an additional boundary check to the test parameter bytes_arg in
generate_random() in test_suite_psa_crypto.function.

This is to check against a possible underflow, and to avoid a warning,
"argument ... exceeds maximum object size", (-Werror=alloc-size-larger-than)
that GCC 7 generates.

Signed-off-by: Simon Butcher <simon.butcher@arm.com>

tests/suites/test_suite_psa_crypto.function

@@ -5297,6 +5297,8 @@ void generate_random( int bytes_arg )
     size_t i;
     unsigned run;
 
+    TEST_ASSERT( bytes_arg >= 0 );
+
     ASSERT_ALLOC( output, bytes + sizeof( trail ) );
     ASSERT_ALLOC( changed, bytes );
     memcpy( output + bytes, trail, sizeof( trail ) );