Fix integer overflow when reading ZIP central directory

Our code didn't convert 32-bit offset to (possibly 64-bit) m_Offset
correctly in the first place, and didn't check if the offset remained
valid after adjustment.

Fix both problems by using explicit cast and checking for the latter
explicitly.

Credit to OSS-Fuzz: this solves its issue 20527.

src/common/zipstrm.cpp

@@ -1852,8 +1852,20 @@ wxStreamError wxZipInputStream::ReadCentral()
     m_position += size;
     m_signature = ReadSignature();
 
-    if (m_offsetAdjustment)
-        m_entry.SetOffset(m_entry.GetOffset() + m_offsetAdjustment);
+    if (m_offsetAdjustment) {
+        // Offset read from the stream is 4 bytes independently of the
+        // platform, but it's not clear if it can become greater than max
+        // 32-bit value after adjustment. For now consider that it can't.
+        wxFileOffset ofs = wxUint32(m_entry.GetOffset());
+        ofs += m_offsetAdjustment;
+        if (ofs > wxUINT32_MAX) {
+            m_signature = 0;
+            return wxSTREAM_READ_ERROR;
+        }
+
+        m_entry.SetOffset(ofs);
+    }
+
     m_entry.SetKey(m_entry.GetOffset());
 
     return wxSTREAM_NO_ERROR;