libmtp crashes if locale is set to a country that uses a comma for decimal separator.

[codestation]

The function VitaMTP_Data_Metadata_To_XML formats a float number to a string then it tries to replace the dot in the decimal by a comma. If the application who loads libmtp changes the locale to one where it uses a comma for decimal separator then the library crashes since it doesn't check that the result of strchr returns a valid pointer (there is no dot in the formatted number).

Here is a testcase that reproduces the issue:

#include <locale.h>
#include <stdio.h>

int main()
{
    setlocale(LC_ALL, "de_DE.UTF-8");
    printf("LC_ALL: %s\n", setlocale(LC_ALL, NULL));
    printf("LC_CTYPE: %s\n", setlocale(LC_CTYPE, NULL));
    printf("%.6f\n", 1.0f);

    setlocale(LC_ALL, "C");
    printf("LC_ALL: %s\n", setlocale(LC_ALL, NULL));
    printf("LC_CTYPE: %s\n", setlocale(LC_CTYPE, NULL));
    printf("%.6f\n", 1.0f);
    return 0;
}

The fix is to make a check before trying to change the dot for a comma

if(period)
    *period = ','; // All this to make period a comma, maybe there is an easier way?