gdb/solib: use early return in solib_read_symbols

This reduces the indent a bit, and makes it clearer where the
"interesting" part of the function is.

At the same time, move the comment to solib.h.

Change-Id: I8004895757ce296742bcec929799a0ce6041938b
Approved-By: Tom Tromey <tom@tromey.com>