Fix String::compareTo(const char*) for invalid strings

When comparing an invalid String object to a non-empty char*, this would
erronously return 0 (equal) because of a typo.

This bug also masked three incorrect checks in related testcases. In two
cases, a String was made invalid and then checked to still contain a
value (these were changed to check that the string is invalid) and in
one case the wrong string was checked.

api/String.cpp

@@ -462,7 +462,7 @@ int String::compareTo(const String &s) const
 int String::compareTo(const char *cstr) const
 {
 	if (!buffer || !cstr) {
-		if (cstr && !*cstr) return 0 - *(unsigned char *)cstr;
+		if (cstr && *cstr) return 0 - *(unsigned char *)cstr;
 		if (buffer && len > 0) return *(unsigned char *)buffer;
 		return 0;
 	}

test/src/String/test_String.cpp

@@ -127,7 +127,7 @@ TEST_CASE ("Testing String(const __FlashStringHelper) constructor() with invalid
   char *buffer = NULL;
 
   arduino::String str1(F(buffer));
-  REQUIRE(str1.compareTo("Hello") == 0);
+  REQUIRE_FALSE(str1);
 }
 
 TEST_CASE ("Testing String(StringSumHelper &&) constructor()", "[String-Ctor-13]")
@@ -158,5 +158,5 @@ TEST_CASE ("Testing String(String &&) with move(String &rhs) from larger to smal
   arduino::String str("Hello");
   arduino::String str1("Arduino");
   str = static_cast<arduino::String&&>(str1);
-  REQUIRE(str1.compareTo("Arduino") == 0);
+  REQUIRE(str.compareTo("Arduino") == 0);
 }

test/src/String/test_operators.cpp

@@ -121,12 +121,20 @@ TEST_CASE ("Testing & String::operator = (const String &) with invalid buffer of
   REQUIRE(str1.compareTo(str2) == 0);
 }
 
-TEST_CASE ("Testing & String::operator = (const char *)", "[String-operator+-14]")
+TEST_CASE ("Testing & String::operator = (const char *) with NULL does not leave string unchanged", "[String-operator+-14]")
 {
   char *buffer = NULL;
   arduino::String str("Hello");
   str = buffer;
-  REQUIRE(str.compareTo("Hello") == 0);
+  REQUIRE(str.compareTo("Hello") != 0);
+}
+
+TEST_CASE ("Testing & String::operator = (const char *) with NULL produces invalid string", "[String-operator+-14]")
+{
+  char *buffer = NULL;
+  arduino::String str("Hello");
+  str = buffer;
+  REQUIRE_FALSE(str);
 }
 
 TEST_CASE ("Testing & String::operator = (const String &) with invalid buffer of first string", "[String-operator+-15]")