CDRIVER-2596 buffer underflow in bson_strncpy

Calling bson_strncpy with size 0 would write one byte before the start
of the destination string.

Author: ajdavis

src/libbson/src/bson/bson-string.c

@@ -562,6 +562,10 @@ bson_strncpy (char *dst,       /* IN */
               const char *src, /* IN */
               size_t size)     /* IN */
 {
+   if (size == 0) {
+      return;
+   }
+
 #ifdef _MSC_VER
    strncpy_s (dst, size, src, _TRUNCATE);
 #else

src/libbson/tests/test-string.c

@@ -279,6 +279,9 @@ test_bson_strncpy (void)
    ASSERT_CMPSTR ("foo", buf);
    bson_strncpy (buf, "foobar", sizeof buf);
    ASSERT_CMPSTR ("foob", buf);
+   /* CDRIVER-2596 make sure strncpy with size 0 doesn't write to buf[-1] */
+   bson_strncpy (buf + 1, "z", 0);
+   ASSERT_CMPSTR ("foob", buf);
 }