Add count checks for memrchr,memset and stpncpy

Author: AlyElashram

libc/src/string/memrchr.cpp

@@ -15,7 +15,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(void *, memrchr, (const void *src, int c, size_t n)) {
-  LIBC_CRASH_ON_NULLPTR(src);
+
+  if (n)
+    LIBC_CRASH_ON_NULLPTR(src);
 
   const unsigned char *str = reinterpret_cast<const unsigned char *>(src);
   const unsigned char ch = static_cast<unsigned char>(c);

libc/src/string/memset.cpp

@@ -15,7 +15,9 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(void *, memset, (void *dst, int value, size_t count)) {
-  LIBC_CRASH_ON_NULLPTR(dst);
+  if (count) {
+    LIBC_CRASH_ON_NULLPTR(dst);
+  }
   inline_memset(dst, static_cast<uint8_t>(value), count);
   return dst;
 }

libc/src/string/stpncpy.cpp

@@ -18,8 +18,10 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(char *, stpncpy,
                    (char *__restrict dest, const char *__restrict src,
                     size_t n)) {
-  LIBC_CRASH_ON_NULLPTR(dest);
-  LIBC_CRASH_ON_NULLPTR(src);
+  if (n) {
+    LIBC_CRASH_ON_NULLPTR(dest);
+    LIBC_CRASH_ON_NULLPTR(src);
+  }
   size_t i;
   // Copy up until \0 is found.
   for (i = 0; i < n && src[i] != '\0'; ++i)