StringPrintfV: add missing va_end()

When returning prematurely, va_end() was not called. This is a
potential resource leak. Found with cppcheck 1.67.

src/syncevo/util.cpp

@@ -723,6 +723,7 @@ std::string StringPrintfV(const char *format, va_list ap)
                 if (buffer) {
                     free(buffer);
                 }
+                va_end(aq);
                 return "";
             }
             size = realsize;