Undefine NAN before defining it again

After upgrading libc++, math.h happens to be included at the point
where this file tries to define NAN. Redefining a macro to a different
value is an error, but #undef'ing a macro is OK even if the macro isn't
actually defined.

Bug: b/175635923
Test: treehugger
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:dd390faa2fcb49ce0e0ebc6ef180118ec370f957)
Merged-In: Ie8e3e87189bcd7ed8fccccfc0a79e04a5bfda16b
Change-Id: Ie8e3e87189bcd7ed8fccccfc0a79e04a5bfda16b