acpigen: Fix ?: operator confusion

strlen(string) was on the "negative" side of the selection operator, the
side where string is NULL.

Change-Id: Ic421a5406ef788c504e30089daeba61a195457ae
Reported-by: Coverity Scan (CID 1355263)
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/14867
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)
Reviewed-by: Duncan Laurie <dlaurie@google.com>

src/arch/x86/acpigen.c

@@ -206,7 +206,7 @@ void acpigen_emit_stream(const char *data, int size)
 
 void acpigen_emit_string(const char *string)
 {
-	acpigen_emit_stream(string, string ? 0 : strlen(string));
+	acpigen_emit_stream(string, string ? strlen(string) : 0);
 	acpigen_emit_byte('\0'); /* NUL */
 }