Fix null pointer check accidentally disabled (#796293)

Compiling (with new enough g++) produces this warning:

    PasswordRAMStore.cc: In member function 'void GParted::PWStore::erase_all()':
    PasswordRAMStore.cc:177:2: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
      if ( protected_mem != NULL );
      ^~
    PasswordRAMStore.cc:193:3: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
       memset( protected_mem, '\0', ProtectedMemSize );
       ^~~~~~

Looks like a stray semicolon...

Bug 796293 - Fix null pointer check accidentally disabled