[scudo] Fix String DCHECK

[cryptoad]

Summary

This resolves an issue tripping a DCHECK, as I was checking for the
capacity and not the size. We don't need to 0-init the Vector as it's
done already, and make sure we only 0-out the string on clear if it's
not empty.

[hctim]

LGTM

compiler-rt/lib/scudo/standalone/string_utils.h

 #include "vector.h"
 
 #include <stdarg.h>
 
 namespace scudo {
 
 class ScopedString {
 public:
-  explicit ScopedString() : String() {
-    if (String.capacity() > 0)
-      String[0] = '\0';
-  }
+  explicit ScopedString() : String() {}
   uptr length() { return Length; }
   const char *data() { return String.data(); }
   void clear() {
+    if (!String.empty())
       String[0] = '\0';
     Length = 0;
   }
   void append(const char *Format, va_list Args);
   void append(const char *Format, ...);
   void output() const { outputRaw(String.data()); }
 
 private:
   Vector<char> String;