Mark a number of coverity false positives.

Fixes coverity #23284, 23285, 23286, 23287, 23289.

src/Length.cpp

@@ -355,6 +355,8 @@ GlueLength::GlueLength(Length const & len, Length const & plus,
 
 GlueLength::GlueLength(string const & data)
 {
+	// false positive from coverity
+	// coverity[CHECKED_RETURN]
 	isValidGlueLength(data, this);
 }
 

src/TextClass.cpp

@@ -528,6 +528,8 @@ TextClass::ReturnValues TextClass::read(Lexer & lexrc, ReadType rt)
 			// Either way, we just scan the rest and discard it
 			else {
 				Layout lay;
+				// false positive from coverity
+				// coverity[CHECKED_RETURN]
 				readStyle(lexrc, lay);
 			}
 			break;

src/VCBackend.cpp

@@ -566,6 +566,8 @@ void CVS::scanMaster()
 			// Ok extract the fields.
 			smatch sm;
 
+			// false positive from coverity
+			// coverity[CHECKED_RETURN]
 			regex_match(line, sm, reg);
 
 			//sm[0]; // whole matched string

src/insets/ExternalTransforms.cpp

@@ -310,6 +310,8 @@ string const sanitizeLatexOption(string const & input)
 	// Strip any trailing commas
 	// "...foo,,,]" -> "...foo" ("...foo,,," may be empty)
 	static lyx::regex const back("^(.*[^,])?,*\\] *$");
+	// false positive from coverity
+	// coverity[CHECKED_RETURN]
 	regex_match(output, what, back);
 	if (!what[0].matched) {
 		lyxerr << "Unable to sanitize LaTeX \"Option\": "

src/lyxfind.cpp

@@ -1408,6 +1408,8 @@ static void findAdvReplace(BufferView * bv, FindAndReplaceOptions const & opt, M
 		docstring repl_latex = ods.str();
 		LYXERR(Debug::FIND, "Latexified replace_buffer: '" << repl_latex << "'");
 		string s;
+		// false positive from coverity
+		// coverity[CHECKED_RETURN]
 		regex_replace(to_utf8(repl_latex), s, "\\$(.*)\\$", "$1");
 		regex_replace(s, s, "\\\\\\[(.*)\\\\\\]", "$1");
 		repl_latex = from_utf8(s);