msvcrt: Update source pointer in wcsrtombs in C locale.

dlls/msvcrt/tests/string.c

@@ -2094,6 +2094,22 @@ static void test_mbstowcs(void)
     ok(ret == 0, "wcstombs did not return 0, got %d\n", (int)ret);
     ok(!mOut[0], "mOut = %s\n", mOut);
 
+    if(pwcsrtombs) {
+        pwstr = wSimple;
+        err = -3;
+        ret = pwcsrtombs(mOut, &pwstr, 4, &err);
+        ok(ret == 4, "wcsrtombs did not return 4\n");
+        ok(err == 0, "err = %d\n", err);
+        ok(pwstr == wSimple+4, "pwstr = %p (wszSimple = %p)\n", pwstr, wSimple);
+        ok(!memcmp(mOut, mSimple, ret), "mOut = %s\n", mOut);
+
+        pwstr = wSimple;
+        ret = pwcsrtombs(mOut, &pwstr, 5, NULL);
+        ok(ret == 4, "wcsrtombs did not return 4\n");
+        ok(pwstr == NULL, "pwstr != NULL\n");
+        ok(!memcmp(mOut, mSimple, sizeof(mSimple)), "mOut = %s\n", mOut);
+    }
+
     if(!setlocale(LC_ALL, "Japanese_Japan.932")) {
         win_skip("Japanese_Japan.932 locale not available\n");
         return;

dlls/msvcrt/wcs.c

@@ -610,6 +610,9 @@ static size_t wcsrtombs_l(char *mbstr, const wchar_t **wcstr,
             mbstr[i] = (*wcstr)[i];
             if(!(*wcstr)[i]) break;
         }
+
+        if(i < count) *wcstr = NULL;
+        else *wcstr += i;
         return i;
     }