fix bad path allocation in makeinfo

fix bad path allocation in makeinfo

In one branch of the image handling, the wrong malloc() call was
copied-pasted making the allocation possibly smaller than needed.
Spotted while building the gcc11 port, libgccjit.info exercise that
branch and the write out of bounds was discovered by malloc().

Convert the malloc + strlcpy/cat dance to an asprintf, and while here
also fix the leak of the two paths.

ok tb@