xcursormgr: avoid scanning ill formed inherit

avoid adding ill formed Inherit lines to inherit vector and later
scanning them, it wont change anything in practice but makes the inherit
theme parsing more in line with what its supposed todo. also check for
return values of the various string functions so we dont end up erasing
the wrong thing.

src/managers/XCursorManager.cpp

@@ -205,23 +205,56 @@ std::unordered_set<std::string> CXCursorManager::themePaths(std::string const& t
         Debug::log(LOG, "XCursor parsing index.theme {}", indexTheme);
 
         while (std::getline(infile, line)) {
+            if (line.empty())
+                continue;
+
             // Trim leading and trailing whitespace
-            line.erase(0, line.find_first_not_of(" \t\n\r"));
-            line.erase(line.find_last_not_of(" \t\n\r") + 1);
+            auto pos = line.find_first_not_of(" \t\n\r");
+            if (pos != std::string::npos)
+                line.erase(0, pos);
+
+            pos = line.find_last_not_of(" \t\n\r");
+            if (pos != std::string::npos && pos < line.length()) {
+                line.erase(pos + 1);
+            }
+
+            if (line.rfind("Inherits", 8) != std::string::npos) { // Check if line starts with "Inherits"
+                std::string inheritThemes = line.substr(8);       // Extract the part after "Inherits"
+                if (inheritThemes.empty())
+                    continue;
 
-            if (line.rfind("Inherits", 0) == 0) {           // Check if line starts with "Inherits"
-                std::string inheritThemes = line.substr(8); // Extract the part after "Inherits"
                 // Remove leading whitespace from inheritThemes and =
-                inheritThemes.erase(0, inheritThemes.find_first_not_of(" \t\n\r"));
-                inheritThemes.erase(0, 1);
-                inheritThemes.erase(0, inheritThemes.find_first_not_of(" \t\n\r"));
+                pos = inheritThemes.find_first_not_of(" \t\n\r");
+                if (pos != std::string::npos)
+                    inheritThemes.erase(0, pos);
+
+                if (inheritThemes.empty())
+                    continue;
+
+                if (inheritThemes.at(0) == '=')
+                    inheritThemes.erase(0, 1);
+                else
+                    continue; // not correct formatted index.theme
+
+                pos = inheritThemes.find_first_not_of(" \t\n\r");
+                if (pos != std::string::npos)
+                    inheritThemes.erase(0, pos);
 
                 std::stringstream inheritStream(inheritThemes);
                 std::string       inheritTheme;
                 while (std::getline(inheritStream, inheritTheme, ',')) {
+                    if (inheritTheme.empty())
+                        continue;
+
                     // Trim leading and trailing whitespace from each theme
-                    inheritTheme.erase(0, inheritTheme.find_first_not_of(" \t\n\r"));
-                    inheritTheme.erase(inheritTheme.find_last_not_of(" \t\n\r") + 1);
+                    pos = inheritTheme.find_first_not_of(" \t\n\r");
+                    if (pos != std::string::npos)
+                        inheritTheme.erase(0, pos);
+
+                    pos = inheritTheme.find_last_not_of(" \t\n\r");
+                    if (pos != std::string::npos && pos < inheritTheme.length())
+                        inheritTheme.erase(inheritTheme.find_last_not_of(" \t\n\r") + 1);
+
                     themes.push_back(inheritTheme);
                 }
             }