In btrace_pt_readmem_callback, we read memory inside TRY/CATCH and return in
case of an error return value. This corrupts the cleanup chain, which
eventually results in a SEGV when doing or discarding cleanups later on.
gdb/
* btrace.c (btrace_pt_readmem_callback): Do not return in TRY/CATCH.
testsuite/
* gdb.btrace/dlopen.exp: New.
* gdb.btrace/dlopen.c: New.
* gdb.btrace/dlopen-dso.c: New.
