cmCacheManager: Prevent corruption from multiline help strings

Corruption happens when a multiline help string starts with
a newline. This causes an overflow in substr() and the whole,
unescaped help string is written to the cache file.

Multiline strings are escaped line by line. The fix is to start
searching for line break points from position 1. A break point
at zero, which triggers the overflow, could only produce
a redundant empty line.

Fixes: #9098

Source/cmCacheManager.cxx

diff --git a/Source/cmCacheManager.cxx b/Source/cmCacheManager.cxx
index 4ce2b66..0a4ed4a 100644
--- a/Source/cmCacheManager.cxx
+++ b/Source/cmCacheManager.cxx
@@ -414,7 +414,7 @@
   }
   std::string oneLine;
   std::string::size_type pos = 0;
-  for (std::string::size_type i = 0; i <= end; i++) {
+  for (std::string::size_type i = 1; i <= end; i++) {
     if ((i == end) || (helpString[i] == '\n') ||
         ((i - pos >= 60) && (helpString[i] == ' '))) {
       fout << "//";

Tests/RunCMake/Configure/CacheVarHelpString.cmake

diff --git a/Tests/RunCMake/Configure/CacheVarHelpString.cmake b/Tests/RunCMake/Configure/CacheVarHelpString.cmake
new file mode 100644
index 0000000..262e527
--- /dev/null
+++ b/Tests/RunCMake/Configure/CacheVarHelpString.cmake
@@ -0,0 +1,21 @@
+# Various newline possibilities
+
+set(help_strings
+  "\n"
+  "\n\n\n"
+  "\n \n"
+  "\nline1"
+  "line1\n"
+  "\nline1\n"
+  "line1\nline2\nline3"
+  "\nline1\nline2\nline3\n"
+  "
+line line line line line line line line line line line line line line line line
+line line line line line line line line line line line line line line line line
+line line line line line line line line line line line line line line line line"
+)
+
+foreach(help IN LISTS help_strings)
+  string(SHA1 name "${help}")
+  set("${name}" "" CACHE STRING "${help}" FORCE)
+endforeach()

Tests/RunCMake/Configure/RunCMakeTest.cmake

diff --git a/Tests/RunCMake/Configure/RunCMakeTest.cmake b/Tests/RunCMake/Configure/RunCMakeTest.cmake
index 9b686e4..57cf947 100644
--- a/Tests/RunCMake/Configure/RunCMakeTest.cmake
+++ b/Tests/RunCMake/Configure/RunCMakeTest.cmake
@@ -64,6 +64,12 @@
   run_cmake(NoCMAKE_DEFAULT_CONFIGS)
 endif()
 
+block()
+  run_cmake(CacheVarHelpString)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  run_cmake(CacheVarHelpString)
+endblock()
+
 if(NOT CMAKE_HOST_WIN32)
   block()
     # Test a non-writable build directory.