Fix segfault in BtreeTest.begin_end_size_exists_find_empty on GCC 5
Fix segfault in BtreeTest.begin_end_size_exists_find_empty on GCC 5

The segfault happens when vector keys is resized and the strings inside are
moved. In this case, the original keys[i].c_str() passed to construct the
BTreeEntry becomes dangling. Reserve enough capacity for the vector to avoid
resizing.

utah-scs/RAMCloud,SMatsushi/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,anirajk/RAMCloud,jblomer/ramcloud,anirajk/RAMCloud,jblomer/ramcloud,jblomer/ramcloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,jblomer/ramcloud,SMatsushi/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,jblomer/ramcloud,anirajk/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,utah-scs/RAMCloud,jblomer/ramcloud