fix a few bugs in FBString

fix a few bugs in FBString

Summary:
* `push_back()` fails for medium strings of zero capacity if not linked
with jemalloc (see added test),
* we incorrectly initiliaze capacity when acquire mallocated string
(forgot about null terminator).

Test Plan: fbconfig --allocator=malloc folly/test:fbstring_test_using_jemalloc && fbmake runtests_opt

@override-unit-failures

Reviewed By: tudorb@fb.com

FB internal diff: D1012196