Avoid eating empty lines in spec %prep section (RhBug:573339)

- In spec %prep context empty lines don't usually matter but they can
  be significant in eg here-documents.
- Fixes regression from commit 94ff22b129

build/parsePrep.c

@@ -522,7 +522,7 @@ int parsePrep(rpmSpec spec)
 	}
     }
 
-    argvSplit(&saveLines, getStringBuf(sb), "\n");
+    saveLines = argvSplitString(getStringBuf(sb), "\n", ARGV_NONE);
     for (lines = saveLines; *lines; lines++) {
 	res = 0;
 	if (rstreqn(*lines, "%setup", sizeof("%setup")-1)) {