msvcrt: Fix mbtowc behavior for buffers containing more than one character.

dlls/msvcrt/mbcs.c

@@ -2157,17 +2157,25 @@ int CDECL MSVCRT_mbtowc_l(MSVCRT_wchar_t *dst, const char* str, MSVCRT_size_t n,
 
     if(n <= 0 || !str)
         return 0;
-    if(!locinfo->lc_codepage)
-        tmpdst = (unsigned char)*str;
-    else if(!MultiByteToWideChar(locinfo->lc_codepage, 0, str, n, &tmpdst, 1))
-        return -1;
-    if(dst)
-        *dst = tmpdst;
-    /* return the number of bytes from src that have been used */
-    if(!*str)
+
+    if(!*str) {
+        if(dst) *dst = 0;
         return 0;
-    if(n >= 2 && MSVCRT__isleadbyte_l((unsigned char)*str, locale) && str[1])
+    }
+
+    if(!locinfo->lc_codepage) {
+        if(dst) *dst = (unsigned char)*str;
+        return 1;
+    }
+    if(n>=2 && MSVCRT__isleadbyte_l((unsigned char)*str, locale)) {
+        if(!MultiByteToWideChar(locinfo->lc_codepage, 0, str, 2, &tmpdst, 1))
+            return -1;
+        if(dst) *dst = tmpdst;
         return 2;
+    }
+    if(!MultiByteToWideChar(locinfo->lc_codepage, 0, str, 1, &tmpdst, 1))
+        return -1;
+    if(dst) *dst = tmpdst;
     return 1;
 }