Avoid mixing code and declarations in C expat code

Some compilers, notably MSVC, don't support C99 and C89 doesn't allow this.

Fixes MSVS compilation after 02da4c8b1d.

(cherry picked from commit c7a3785a5c)

src/expat/lib/xmlparse.c

@@ -1703,13 +1703,16 @@ XML_GetBuffer(XML_Parser parser, int len)
   }
 
   if (len > bufferLim - bufferEnd) {
+#ifdef XML_CONTEXT_BYTES
+    int keep;
+#endif
     int neededSize = len + (int)(bufferEnd - bufferPtr);
     if (neededSize < 0) {
       errorCode = XML_ERROR_NO_MEMORY;
       return NULL;
     }
 #ifdef XML_CONTEXT_BYTES
-    int keep = (int)(bufferPtr - buffer);
+    keep = (int)(bufferPtr - buffer);
 
     if (keep > XML_CONTEXT_BYTES)
       keep = XML_CONTEXT_BYTES;