LibC: Add strnlen()

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/60143c8d4e4

Libraries/LibC/string.cpp

@@ -55,6 +55,14 @@ size_t strlen(const char* str)
     return len;
 }
 
+size_t strnlen(const char* str, size_t maxlen)
+{
+    size_t len = 0;
+    for (; len < maxlen && *str; str++)
+        len++;
+    return len;
+}
+
 char* strdup(const char* str)
 {
     size_t len = strlen(str);

Libraries/LibC/string.h

@@ -6,6 +6,7 @@
 __BEGIN_DECLS
 
 size_t strlen(const char*);
+size_t strnlen(const char*, size_t maxlen);
 int strcmp(const char*, const char*);
 int strncmp(const char*, const char*, size_t);
 int strcasecmp(const char*, const char*);