InMemMsgLoader::loadMsg(): fix memory leak when transcoding fails.

Seen with the IconvGNU transcoder when parsing "<aaa.xsdopengis.net/gml\x96".
The reason is that XMLString::transcode(repText2, manager) throws a TranscodingException
which causes the tmp1 string to leak.

```
0 0x8791409 in operator new(unsigned int) /src/llvm-project/compiler-rt/lib/asan/asan_new_delete.cpp:99:3
1 0xbd147f7 in xercesc_4_0::MemoryManagerImpl::allocate(unsigned int) gdal/xerces-c/src/xercesc/internal/MemoryManagerImpl.cpp:40:18
2 0xbe8c73e in xercesc_4_0::IconvGNULCPTranscoder::transcode(char const*, xercesc_4_0::MemoryManager*) gdal/xerces-c/src/xercesc/util/Transcoders/IconvGNU/IconvGNUTransService.cpp:870:32
3 0xbc22ca2 in xercesc_4_0::XMLString::transcode(char const*, xercesc_4_0::MemoryManager*) gdal/xerces-c/src/xercesc/util/XMLString.cpp:621:25
4 0xbe8f4ad in xercesc_4_0::InMemMsgLoader::loadMsg(unsigned int, char16_t*, unsigned int, char const*, char const*, char const*, char const*, xercesc_4_0::MemoryManager*) gdal/xerces-c/src/xercesc/util/MsgLoaders/InMemory/InMemMsgLoader.cpp:157:16
5 0xbc20175 in xercesc_4_0::XMLException::loadExceptText(xercesc_4_0::XMLExcepts::Codes, char const*, char const*, char const*, char const*) gdal/xerces-c/src/xercesc/util/XMLException.cpp:241:23
6 0xbc48bee in xercesc_4_0::UTFDataFormatException::UTFDataFormatException(char const*, unsigned long long, xercesc_4_0::XMLExcepts::Codes, char const*, char const*, char const*, char const*, xercesc_4_0::MemoryManager*) gdal/xerces-c/src/xercesc/util/UTFDataFormatException.hpp:31:1
7 0xbc4824e in xercesc_4_0::XMLUTF8Transcoder::transcodeFrom(unsigned char const*, unsigned int, char16_t*, unsigned int, unsigned int&, unsigned char*) gdal/xerces-c/src/xercesc/util/XMLUTF8Transcoder.cpp:182:13
8 0xbd27d7e in xercesc_4_0::XMLReader::xcodeMoreChars(char16_t*, unsigned char*, unsigned int) gdal/xerces-c/src/xercesc/internal/XMLReader.cpp:1926:34
9 0xbd271dd in xercesc_4_0::XMLReader::refreshCharBuffer() gdal/xerces-c/src/xercesc/internal/XMLReader.cpp:571:19
10 0xbd15c63 in xercesc_4_0::XMLReader::peekNextChar(char16_t&) gdal/xerces-c/src/xercesc/internal/XMLReader.hpp:767:14
11 0xbd15aaf in xercesc_4_0::ReaderMgr::peekNextChar() gdal/xerces-c/src/xercesc/internal/ReaderMgr.cpp:158:21
12 0xbd328da in xercesc_4_0::XMLScanner::scanProlog() gdal/xerces-c/src/xercesc/internal/XMLScanner.cpp:1241:45
13 0xbd31ef4 in xercesc_4_0::XMLScanner::scanFirst(xercesc_4_0::InputSource const&, xercesc_4_0::XMLPScanToken&) gdal/xerces-c/src/xercesc/internal/XMLScanner.cpp:549:9
14 0xbdadcff in xercesc_4_0::SAX2XMLReaderImpl::parseFirst(xercesc_4_0::InputSource const&, xercesc_4_0::XMLPScanToken&) gdal/xerces-c/src/xercesc/parsers/SAX2XMLReaderImpl.cpp:500:22
```

src/xercesc/util/MsgLoaders/InMemory/InMemMsgLoader.cpp

@@ -25,6 +25,7 @@
 // ---------------------------------------------------------------------------
 #include <xercesc/util/BitOps.hpp>
 #include <xercesc/util/PlatformUtils.hpp>
+#include <xercesc/util/TranscodingException.hpp>
 #include <xercesc/util/XMLMsgLoader.hpp>
 #include <xercesc/util/XMLString.hpp>
 #include <xercesc/util/XMLUni.hpp>
@@ -153,14 +154,28 @@ bool InMemMsgLoader::loadMsg(const  XMLMsgLoader::XMLMsgId  msgToLoad
     XMLCh* tmp4 = 0;
 
     bool bRet = false;
-    if (repText1)
-        tmp1 = XMLString::transcode(repText1, manager);
-    if (repText2)
-        tmp2 = XMLString::transcode(repText2, manager);
-    if (repText3)
-        tmp3 = XMLString::transcode(repText3, manager);
-    if (repText4)
-        tmp4 = XMLString::transcode(repText4, manager);
+    try
+    {
+        if (repText1)
+            tmp1 = XMLString::transcode(repText1, manager);
+        if (repText2)
+            tmp2 = XMLString::transcode(repText2, manager);
+        if (repText3)
+            tmp3 = XMLString::transcode(repText3, manager);
+        if (repText4)
+            tmp4 = XMLString::transcode(repText4, manager);
+    }
+    catch( const TranscodingException& )
+    {
+        if (tmp1)
+            manager->deallocate(tmp1);
+        if (tmp2)
+            manager->deallocate(tmp2);
+        if (tmp3)
+            manager->deallocate(tmp3);
+        // Note: tmp4 cannot leak
+        throw;
+    }
 
     bRet = loadMsg(msgToLoad, toFill, maxChars, tmp1, tmp2, tmp3, tmp4, manager);