util/FormatString: pass the allocated buffer to AllocatedString::Donate()

.. and not the stack buffer.  This made the AllocatedString destructor
crash.

Closes #52

NEWS

@@ -3,6 +3,7 @@ ver 0.20.10 (not yet released)
   - ffmpeg: support MusicBrainz ID3v2 tags
 * tags
   - aiff: fix FORM chunk size endianess (is big-endian)
+* fix crash on Windows
 
 ver 0.20.9 (2017/06/04)
 * decoder

src/util/FormatString.cxx

@@ -57,7 +57,7 @@ FormatStringV(const char *fmt, va_list args)
 	const size_t length = strlen(buffer);
 	char *p = new char[length + 1];
 	memcpy(p, buffer, length + 1);
-	return AllocatedString<>::Donate(buffer);
+	return AllocatedString<>::Donate(p);
 #endif
 }