ICU-23243 Bugfix: Don't call std::string_view::front() when empty.

Doing this just happens to work in most builds (which is why this bug
hasn't been found until now), but it's actually undefined behavior.

Author: roubert

icu4c/source/common/uloc.cpp

@@ -627,7 +627,7 @@ ulocimp_getKeywords(std::string_view localeID,
         do {
             bool duplicate = false;
             /* skip leading spaces */
-            while (localeID.front() == ' ') {
+            while (!localeID.empty() && localeID.front() == ' ') {
                 localeID.remove_prefix(1);
             }
             if (localeID.empty()) { /* handle trailing "; " */

icu4c/source/test/intltest/loctest.cpp

@@ -266,6 +266,7 @@ void LocaleTest::runIndexedTest( int32_t index, UBool exec, const char* &name, c
     TESTCASE_AUTO(TestBug13554);
     TESTCASE_AUTO(TestBug20410);
     TESTCASE_AUTO(TestBug20900);
+    TESTCASE_AUTO(TestChromiumBug451657601);
     TESTCASE_AUTO(TestLocaleCanonicalizationFromFile);
     TESTCASE_AUTO(TestKnownCanonicalizedListCorrect);
     TESTCASE_AUTO(TestConstructorAcceptsBCP47);
@@ -5849,6 +5850,12 @@ void LocaleTest::TestBug20900() {
     }
 }
 
+void LocaleTest::TestChromiumBug451657601() {
+    // This used to cause a crash in _LIBCPP_HARDENING_MODE.
+    Locale l = Locale("@x=@; ");
+    assertEquals("canonicalized", "@x=@", l.getName());
+}
+
 U_DEFINE_LOCAL_OPEN_POINTER(LocalStdioFilePointer, FILE, fclose);
 void LocaleTest::TestLocaleCanonicalizationFromFile()
 {

icu4c/source/test/intltest/loctest.h

@@ -126,6 +126,7 @@ class LocaleTest: public IntlTest {
     void TestBug13554();
     void TestBug20410();
     void TestBug20900();
+    void TestChromiumBug451657601();
     void TestLocaleCanonicalizationFromFile();
     void TestKnownCanonicalizedListCorrect();
     void TestConstructorAcceptsBCP47();