Avoid -Werror=format-truncation=

...as emitted by at least GCC 8.2 with --enable-optimized, about trying to
output up to 4 bytes into a destionation of size 3

Change-Id: I42c3c9bb7a0355a7d4a1574c6c65d6f74a97f1dc
Reviewed-on: https://gerrit.libreoffice.org/66617
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

forms/source/xforms/submission/serialization_urlencoded.cxx

@@ -73,7 +73,7 @@ void  CSerializationURLEncoded::encode_and_append(const OUString& aString, OStri
 {
     OString utf8String = OUStringToOString(aString, RTL_TEXTENCODING_UTF8);
     const sal_uInt8 *pString = reinterpret_cast< const sal_uInt8 * >( utf8String.getStr() );
-    sal_Char tmpChar[4]; tmpChar[3] = 0;
+    sal_Char tmpChar[4];
 
     while( *pString != 0)
     {
@@ -89,16 +89,16 @@ void  CSerializationURLEncoded::encode_and_append(const OUString& aString, OStri
             } else if (*pString == 0x0a) {
                 aBuffer.append("%0D%0A");
             } else {
-                snprintf(tmpChar, 3, "%%%X", *pString % 0x100);
+                snprintf(tmpChar, 4, "%%%X", *pString % 0x100);
                 aBuffer.append(tmpChar);
             }
         } else {
-            snprintf(tmpChar, 3, "%%%X", *pString % 0x100);
+            snprintf(tmpChar, 4, "%%%X", *pString % 0x100);
             aBuffer.append(tmpChar);
             while (*pString >= 0x80) {
                 // continuation...
                 pString++;
-                snprintf(tmpChar, 3, "%%%X", *pString % 0x100);
+                snprintf(tmpChar, 4, "%%%X", *pString % 0x100);
                 aBuffer.append(tmpChar);
             }
         }