imm32: Set lengths to 0 for NULL strings in ImmSetCompositionString().

dlls/imm32/imm.c

@@ -2539,6 +2539,9 @@ BOOL WINAPI ImmSetCompositionStringA(
         return FALSE;
 
     if (!(ime = imc_select_ime( data ))) return FALSE;
+    if (!lpComp) dwCompLen = 0;
+    if (!lpRead) dwReadLen = 0;
+
     if (!ime_is_unicode( ime )) return ime->pImeSetCompositionString( hIMC, dwIndex, lpComp, dwCompLen, lpRead, dwReadLen );
 
     comp_len = MultiByteToWideChar(CP_ACP, 0, lpComp, dwCompLen, NULL, 0);
@@ -2596,6 +2599,9 @@ BOOL WINAPI ImmSetCompositionStringW(
         return FALSE;
 
     if (!(ime = imc_select_ime( data ))) return FALSE;
+    if (!lpComp) dwCompLen = 0;
+    if (!lpRead) dwReadLen = 0;
+
     if (ime_is_unicode( ime )) return ime->pImeSetCompositionString( hIMC, dwIndex, lpComp, dwCompLen, lpRead, dwReadLen );
 
     comp_len = WideCharToMultiByte(CP_ACP, 0, lpComp, dwCompLen, NULL, 0, NULL,

dlls/imm32/tests/imm32.c

@@ -1085,12 +1085,24 @@ static void test_SCS_SETSTR(void)
     DWORD prop;
 
     imc = ImmGetContext(hwnd);
-    ret = ImmSetCompositionStringW(imc, SCS_SETSTR, string, sizeof(string), NULL,0);
+    ret = ImmSetCompositionStringW(imc, SCS_SETSTR, string, sizeof(string), NULL, 0);
     if (!ret) {
         win_skip("Composition isn't supported\n");
         ImmReleaseContext(hwnd, imc);
         return;
     }
+
+    ret = ImmSetCompositionStringW(imc, SCS_SETSTR, NULL, 128, NULL, 128);
+    ok(ret, "got error %lu.\n", GetLastError());
+
+    alen = ImmGetCompositionStringA(imc, GCS_COMPSTR, cstring, 20);
+    ok(!alen, "got %ld.\n", alen);
+    wlen = ImmGetCompositionStringW(imc, GCS_COMPSTR, wstring, 20);
+    ok(!wlen, "got %ld.\n", alen);
+
+    ret = ImmSetCompositionStringW(imc, SCS_SETSTR, string, sizeof(string), NULL, 2);
+    ok(ret, "got error %lu.\n", GetLastError());
+
     msg_spy_flush_msgs();
 
     alen = ImmGetCompositionStringA(imc, GCS_COMPSTR, cstring, 20);