gpg: Fix possible dead code elimination.
* g10/encrypt.c: Change condition for detecting a real file.
Detected by Stack 3.0:
bug: anti-dce model: | %tobool155 = icmp ne i32 %call154, 0, !dbg !1298 --> true ************************************************************ land.lhs.true156: %96 = icmp eq i8* %filename, null call void @opt.bugon(i1 %96), !dbg !1298, !bug !1250 %97 = load i8* %filename, align 1, !dbg !1298 %conv157 = sext i8 %97 to i32, !dbg !1298 %tobool158 = icmp ne i32 %conv157, 0, !dbg !1298 br i1 %tobool158, label %land.lhs.true159, label %if.else177,\ !dbg !1298 stack: - /home/wk/s/gnupg/g10/encrypt.c:639:0 ncore: 1 core: - /home/wk/s/gnupg/g10/encrypt.c:639:0 - null pointer dereference