btrace: do not return out of TRY/CATCH

btrace: do not return out of TRY/CATCH

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.