[msan] Add test for deferencing zero-sized malloc/calloc

MSan fails to catch this, because 0-byte allocations are converted into
1-byte allocations.

Bug originally reported by dvyukov

Author: thurstond

compiler-rt/test/msan/zero_alloc.cpp

@@ -0,0 +1,37 @@
+// RUN: %clang_msan -Wno-alloc-size -fsanitize-recover=memory %s -o %t && not %run %t 2>&1 | FileCheck %s
+
+// XFAIL: *
+
+#include <malloc.h>
+#include <stdio.h>
+
+int main(int argc, char **argv) {
+  {
+     char* p1 = (char*)calloc(1, 0);
+     printf ("p1 is %p\n", p1);
+     printf ("Content of p1 is: %d\n", *p1);
+     // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
+     // CHECK: {{#0 0x.* in main .*zero_alloc.cpp:}}[[@LINE-2]]
+     free(p1);
+  }
+
+  {
+     char* p2 = (char*)calloc(0, 1);
+     printf ("p2 is %p\n", p2);
+     printf ("Content of p2 is: %d\n", *p2);
+     // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
+     // CHECK: {{#0 0x.* in main .*zero_alloc.cpp:}}[[@LINE-2]]
+     free(p2);
+  }
+
+  {
+     char* p3 = (char*)malloc(0);
+     printf ("p3 is %p\n", p3);
+     printf ("Content of p2 is: %d\n", *p3);
+     // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
+     // CHECK: {{#0 0x.* in main .*zero_alloc.cpp:}}[[@LINE-2]]
+     free(p3);
+  }
+
+  return 0;
+}