[core/zip] Validate target size before compression

In practice, the target size is greater or equal the source size in most
cases for ROOT, but add this additional correct check to fuzz the inputs
in the next commit.

core/zip/src/RZip.cxx

@@ -78,14 +78,18 @@ unsigned long R__crc32(unsigned long crc, const unsigned char* buf, unsigned int
 /*                      3 = old */
 void R__zipMultipleAlgorithm(int cxlevel, int *srcsize, char *src, int *tgtsize, char *tgt, int *irep, ROOT::RCompressionSetting::EAlgorithm::EValues compressionAlgorithm)
 {
+  *irep = 0;
 
+  // Performance optimization: avoid compressing tiny source buffers.
   if (*srcsize < 1 + HDRSIZE + 1) {
-     *irep = 0;
+     return;
+  }
+  // Correctness check: we need at least enough bytes to prepend the header!
+  if (*tgtsize <= HDRSIZE) {
      return;
   }
 
   if (cxlevel <= 0) {
-    *irep = 0;
     return;
   }