str: provide a zero mechanism

We provide `git__memzero` but make a simple helper for `git_str_zero`
that does the same.

src/util/str.c

@@ -145,6 +145,14 @@ void git_str_clear(git_str *buf)
 		buf->ptr[0] = '\0';
 }
 
+void git_str_zero(git_str *buf)
+{
+	if (buf->asize > 0)
+		git__memzero(buf->ptr, buf->asize);
+
+	git_str_clear(buf);
+}
+
 int git_str_set(git_str *buf, const void *data, size_t len)
 {
 	size_t alloclen;

src/util/str.h

@@ -354,4 +354,12 @@ int git_str_is_binary(const git_str *str);
 */
 int git_str_contains_nul(const git_str *str);
 
+/**
+ * Clears the given string buffer and overwrites it with zeros, to
+ * prevent sensitive data from remaining in the heap.
+ *
+ * @param str string buffer to clear
+ */
+void git_str_zero(git_str *str);
+
 #endif