Add cast to VEC_iterate

Add cast to VEC_iterate

Fixes this in C++:

 ../../src/gdb/break-catch-sig.c: In function ‘int VEC_gdb_signal_type_iterate(const VEC_gdb_signal_type*, unsigned int, gdb_signal_type*)’:
 ../../src/gdb/common/vec.h:576:12: error: invalid conversion from ‘int’ to ‘gdb_signal_type {aka gdb_signal}’ [-fpermissive]
	*ptr = 0;          \
	     ^
 ../../src/gdb/common/vec.h:417:1: note: in expansion of macro ‘DEF_VEC_FUNC_P’
  DEF_VEC_FUNC_P(T)         \
  ^
 ../../src/gdb/break-catch-sig.c:37:1: note: in expansion of macro ‘DEF_VEC_I’
  DEF_VEC_I (gdb_signal_type);
  ^

I actually carried a different fix in the C++ branch that removed this
assignment and then adjusted all callers that depended on it.  The
thinking was that this is for the case where we're returning false,
indicating end of iteration.  But that results in a much larger and
tricker patch; looking back it seems quite pointless.  I looked at the
history of GCC's C++ conversion and saw that they added this same cast
to their version of vec.h, FWIW.  (GCC's vec.h is completely different
nowadays, having been converted to templates meanwhile.)

gdb/ChangeLog:
2015-10-29  Pedro Alves  <palves@redhat.com>

	* common/vec.h (DEF_VEC_FUNC_P) [iterate]: Cast 0 to type T.