Eliminate a buffer size warning in EclSolar::getEclYearInfo()

src/lib/astro/eclsolar.cpp: In member function 'void EclSolar::getEclYearInfo(char*)':
src/lib/astro/eclsolar.cpp:469: warning: '__builtin___sprintf_chk' may write
  a terminating nul past the end of the destination
/usr/include/bits/stdio2.h:38: note: '__builtin___sprintf_chk' output between 5
  and 14 bytes into a destination of size 13