test/common: make sure we a valid str in str_erase

In test_str_erase a buffer (buf[256]) is filled with random data. Later it is duplicated and then erased via bolt_str_erase. As a check it is compared to buf via g_assert_cmpstr and asserted that they are not equal. But if by chance buf[0] is 0, the string is empty and the check will fail, because there is nothing to erase. Fix that by having the random data prefixed with "bolt", so that there is always a valid string to be erased.

Reported-By: Vladimír Čunát v@cunat.cz

Reported-By: Sebastien Bacher seb128@ubuntu.com

Closes #161 (closed)