Remove unnecessary nonnull attribute for snprintf and vsnprintf

According to the C standard, the first argument to snprintf and
vsnprintf may be null if the size argument is zero.

Author: bozicrHT

clang/include/clang/Basic/Builtins.td

@@ -3116,7 +3116,7 @@ def FPrintf : LibBuiltin<"stdio.h"> {
 
 def SnPrintf : LibBuiltin<"stdio.h"> {
   let Spellings = ["snprintf"];
-  let Attributes = [NoThrow, PrintfFormat<2>, NonNull<[0, 2]>];
+  let Attributes = [NoThrow, PrintfFormat<2>, NonNull<[2]>];
   let Prototype = "int(char* restrict, size_t, char const* restrict, ...)";
   let AddBuiltinPrefixedAlias = 1;
 }
@@ -3144,7 +3144,7 @@ def VfPrintf : LibBuiltin<"stdio.h"> {
 
 def VsnPrintf : LibBuiltin<"stdio.h"> {
   let Spellings = ["vsnprintf"];
-  let Attributes = [NoThrow, VPrintfFormat<2>, NonNull<[0, 2]>];
+  let Attributes = [NoThrow, VPrintfFormat<2>, NonNull<[2]>];
   let Prototype = "int(char* restrict, size_t, char const* restrict, __builtin_va_list)";
   let AddBuiltinPrefixedAlias = 1;
 }

clang/test/Sema/format-strings-nonnull.c

@@ -41,9 +41,8 @@ void check_format_string(FILE *fp, va_list ap) {
     // expected-warning@-1{{null passed to a callee that requires a non-null argument}}
     // expected-warning@-2{{null passed to a callee that requires a non-null argument}}
 
-    snprintf(NULL, 10, 0, 42);
+    snprintf(buf, 10, 0, 42);
     // expected-warning@-1{{null passed to a callee that requires a non-null argument}}
-    // expected-warning@-2{{null passed to a callee that requires a non-null argument}}
 
     vprintf(fmt, ap);
     // expected-warning@-1{{null passed to a callee that requires a non-null argument}}
@@ -54,9 +53,8 @@ void check_format_string(FILE *fp, va_list ap) {
     vsprintf(buf, nullptr, ap);
     // expected-warning@-1{{null passed to a callee that requires a non-null argument}}
 
-    vsnprintf(NULL, 10, fmt, ap);
+    vsnprintf(buf, 10, fmt, ap);
     // expected-warning@-1{{null passed to a callee that requires a non-null argument}}
-    // expected-warning@-2{{null passed to a callee that requires a non-null argument}}
 
     scanf(NULL);
     // expected-warning@-1{{null passed to a callee that requires a non-null argument}}