Fix XM save path to skip sample data after sample headers (#1369)

XM saving failed for files with samples because the save logic only
advanced past sample headers and did not skip the sample data, causing
the next instrument to be written at the wrong position.

taglib/xm/xmfile.cpp

@@ -457,6 +457,7 @@ bool XM::File::save()
     }
 
     unsigned long sampleHeaderSize = 0;
+    unsigned long sampleDataSize = 0;
     if(sampleCount > 0) {
       seek(pos + 29);
       if(instrumentHeaderSize < 33U || !readU32L(sampleHeaderSize))
@@ -468,9 +469,12 @@ bool XM::File::save()
     for(unsigned short j = 0; j < sampleCount; ++ j) {
       if(sampleHeaderSize > 4U) {
         seek(pos);
-        if(unsigned long sampleLength = 0; !readU32L(sampleLength))
+        unsigned long sampleLength = 0;
+        if(!readU32L(sampleLength))
           return false;
 
+        sampleDataSize += sampleLength;
+
         if(sampleHeaderSize > 18U) {
           seek(pos + 18);
           const auto sz = std::min(sampleHeaderSize - 18, 22UL);
@@ -482,6 +486,8 @@ bool XM::File::save()
       }
       pos += sampleHeaderSize;
     }
+
+    pos += sampleDataSize;
   }
 
   return true;