Add multithreaded example

tests/regression/79-mutex2/02-split-mt.c

@@ -0,0 +1,37 @@
+#include<pthread.h>
+
+pthread_mutex_t m1;
+pthread_mutex_t m2;
+pthread_mutex_t* ptr;
+
+void other() {
+    int top;
+    ptr = &m2;
+
+    if(top) {
+        ptr = &m1;
+    }
+}
+
+int main(int argc, char const *argv[])
+{
+    int top;
+
+    ptr = &m1;
+
+    if(top) {
+        ptr = &m2;
+    }
+
+    pthread_t mischievous;
+    pthread_create(&mischievous, NULL, other, NULL);
+
+
+    pthread_mutex_lock(ptr);
+
+    // This has to produce a warning, as the other thread may have changed what
+    // ptr points to such that it's not the same mutex being unlocked here.
+    pthread_mutex_unlock(ptr); //WARN
+
+    return 0;
+}