Fix uninitialised variable warning

```
[87/375] CC build/release/obj/rdb_protocol/geo/karney/geodesic.o
src/rdb_protocol/geo/karney/geodesic.cc: In function ?real geod_geninverse(const geod_geodesic*, real,
real, real, real, real*, real*, real*, real*, real*, real*, real*)?: src/rdb_protocol/geo/karney/geodesic.cc:784:28:
error: 'dv' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         if (numit < maxit1 && dv > 0) {
                            ^
cc1plus: all warnings being treated as errors
src/build.mk:379: recipe for target 'build/release/obj/rdb_protocol/geo/karney/geodesic.o' failed
make[1]: *** [build/release/obj/rdb_protocol/geo/karney/geodesic.o] Error 1
make[1]: *** Waiting for unfinished jobs....
Makefile:52: recipe for target 'make' failed
make: *** [make] Error 2
```
```
sathya@sathya:~/code/rethinkdb$ clang -v 
Debian clang version 3.4.2-8 (tags/RELEASE_34/dot2-final) (based on LLVM 3.4.2)
```