ctime.3: Add a note about a possible return value of localtime(3)

The localtime(3) function returns a NULL pointer, if the passed in-time
translates to a year that will not fit in an integer type. It is stricly
recommended to check the return value to avoid garage output.

Reported by: mckusick
Reviewed by: mckusick, imp, rpokala
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36515