FT-259 Placate the build slaves with their exotic toolchain.

util/tests/memarena-test.cc

@@ -118,7 +118,9 @@ class memarena_unit_test {
 
         // make sure memory was allocated ok by
         // writing to buf and reading it back
+        if (size > 0) {
             memset(ma._current_chunk.buf, magic, size);
+        }
         for (size_t i = 0; i < size; i++) {
             const char *buf = reinterpret_cast<char *>(ma._current_chunk.buf);
             invariant(buf[i] == magic);
@@ -134,7 +136,9 @@ class memarena_unit_test {
 
         // make sure memory was allocated ok by
         // writing to buf and reading it back
+        if (size > 0) {
             memset(ma._current_chunk.buf, magic, size);
+        }
         for (size_t i = 0; i < size; i++) {
             const char *c = reinterpret_cast<char *>(ma._current_chunk.buf);
             invariant(c[i] == magic);