Fix memory leak in rgbe_header_write.
clang + scanbuild:
[90/1269] Compiling C object libs/rgbe/librgbe.a.p/rgbe.c.o
../../../libs/rgbe/rgbe.c:836:10: warning: Potential leak of memory pointed to by 'line' [unix.Malloc]
return success;
^~~~~~~
Edited by Øyvind "pippin" Kolås