[RESTRICT AUTOMERGE] 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
Change-Id: Ie8e3e87189bcd7ed8fccccfc0a79e04a5bfda16b
Merged-In: Ie8e3e87189bcd7ed8fccccfc0a79e04a5bfda16b