Fix integer overflow in ZIP reading code

Check for the record size before subtracting it from the end position:
the former must be smaller than the latter for all valid ZIP files and
not performing this check could result in an integer overflow error from
the undefined behaviour sanitizer for bad input.

Credit to OSS-Fuzz: this solves its issue 3828.

src/common/zipstrm.cpp

@@ -1678,7 +1678,16 @@ bool wxZipInputStream::LoadEndRecord()
     wxZipEndRec endrec;
 
     // Read in the end record
-    wxFileOffset endPos = m_parent_i_stream->TellI() - 4;
+    const wxFileOffset curPos = m_parent_i_stream->TellI();
+    if ( curPos < 4 )
+    {
+        // Either failed to get the position (if it's negative) or can't be a
+        // valid ZIP file anyhow (this probably can't happen here, but it
+        // doesn't harm to check).
+        return false;
+    }
+
+    const size_t endPos = curPos - 4;
     if (!endrec.Read(*m_parent_i_stream, GetConv()))
         return false;
 
@@ -1700,10 +1709,12 @@ bool wxZipInputStream::LoadEndRecord()
     // If it's not there, then it could be that the zip has been appended
     // to a self extractor, so take the CD size (also in endrec), subtract
     // it from the file offset of the end-central-directory and look there.
-    if (m_parent_i_stream->SeekI(endPos - endrec.GetSize())
-            != wxInvalidOffset && ReadSignature() == magic) {
+    const wxUint64 recSize = endrec.GetSize();
+    if (recSize <= endPos &&
+            m_parent_i_stream->SeekI(endPos - recSize) != wxInvalidOffset &&
+                ReadSignature() == magic) {
         m_signature = magic;
-        m_position = endPos - endrec.GetSize();
+        m_position = endPos - recSize;
         m_offsetAdjustment = m_position - endrec.GetOffset();
         return true;
     }