Avoid undefined behavior in gdbscm_make_breakpoint

Avoid undefined behavior in gdbscm_make_breakpoint

Running gdb.guile/scm-breakpoint.exp against an --enable-ubsan build,
we see:

 UNRESOLVED: gdb.guile/scm-breakpoint.exp: test_watchpoints: create a breakpoint with an invalid type number
 ...
 guile (define wp2 (make-breakpoint "result" #:wp-class WP_WRITE #:type 999))
 ../../src/gdb/guile/scm-breakpoint.c:377:11: runtime error: load of value 999, which is not a valid value for type 'bptype'
 ERROR: GDB process no longer exists

Fix this by parsing the user/guile input as plain int, and cast to
internal type only after we know we have a number that would be valid.

Change-Id: I03578d07db00be01b610a8f5ce72e5521aea6a4b