[sanitizer] Fix mallopt test on Android.

There is not a single common mallopt option between gnu/linux and
android, so simply use a random number there.

llvm-svn: 345394

compiler-rt/test/sanitizer_common/TestCases/Linux/mallopt.cc

@@ -4,6 +4,7 @@
 #include <malloc.h>
 
 int main() {
-  int res = mallopt(M_ARENA_MAX, 0);
+  // Try a random mallopt option, possibly invalid.
+  int res = mallopt(-42, 0);
   assert(res == 0 || res == 1);
 }