Fix crasher caused by setting a bit in a possibly empty bitvector while
doing printf format string checking.  This is a recent regression.

llvm-svn: 97318
diff --git a/clang/test/Sema/format-strings.c b/clang/test/Sema/format-strings.c
index e92e17d..21d3aec 100644
--- a/clang/test/Sema/format-strings.c
+++ b/clang/test/Sema/format-strings.c
@@ -145,6 +145,7 @@
 }
 
 void test10(int x, float f, int i, long long lli) {
+  printf("%s"); // expected-warning{{more '%' conversions than data arguments}}
   printf("%@", 12); // expected-warning{{invalid conversion specifier '@'}}
   printf("\0"); // expected-warning{{format string contains '\0' within the string body}}
   printf("xs\0"); // expected-warning{{format string contains '\0' within the string body}}