LibC: Always assign the offset pointer to endptr in strto{u,}ll()

This patch makes strto{u,}l{l,}() behave more to-spec about endptr.
"If endptr is not NULL, strtoull stores the address of the first invalid
character in *endptr."

Libraries/LibC/stdlib.cpp

@@ -929,6 +929,9 @@ long long strtoll(const char* str, char** endptr, int base)
         return 0;
     }
 
+    if (endptr)
+        *endptr = parse_ptr;
+
     if (overflow) {
         errno = ERANGE;
         if (sign != Sign::Negative) {
@@ -1003,6 +1006,9 @@ unsigned long long strtoull(const char* str, char** endptr, int base)
         return 0;
     }
 
+    if (endptr)
+        *endptr = parse_ptr;
+
     if (overflow) {
         errno = ERANGE;
         return LONG_LONG_MAX;