harden gbfgetcstr, gbfgetpstr. (#782)

harden gbfgetcstr, gbfgetpstr. (#782)

Hangs were detected with afl and the gdb reader and gbfgetcstr.  One such
hang reads a fuzzed 32 bit url count with FREAD_i32, and then proceeds
to fetch that many strings with FREAD_CSTR_AS_QSTR.  This results in
fetches from beyond EOF that are undetected.  gbfgetcstr_old is modified
to throw a fatal error if reading is attempted beyond EOF.

A similar failure was not detected with gbfgetpstr, but it is also
modified to throw a fatal error if reading is attempted beyond EOF.