Fixes preprocessor regression causing hang

Cppcheck 1.76 introduced a regression in preprocessor which causes
the following code to hang:

    $ cat > test.c << EOF
    #ifndef Y
    #else
    #endif
    EOF
    $ cppcheck -D BAR --force test.c
    Checking test.c ...
    ^C

This used to work with version 1.75. Git bisect reveals that this
regression was caused by commit:
ff036c8742

This commit fixes the regression by avoiding infinite loop in
hasDefine(). If cfg is empty string "", we can skip the whole loop
and exit early.

lib/preprocessor.cpp

@@ -217,6 +217,10 @@ static std::string readcondition(const simplecpp::Token *iftok, const std::set<s
 
 static bool hasDefine(const std::string &userDefines, const std::string &cfg)
 {
+    if (cfg.empty()) {
+        return false;
+    }
+
     std::string::size_type pos = 0;
     while (pos < userDefines.size()) {
         pos = userDefines.find(cfg, pos);

test/testpreprocessor.cpp

@@ -224,6 +224,7 @@ private:
         TEST_CASE(getConfigsU4);
         TEST_CASE(getConfigsU5);
         TEST_CASE(getConfigsU6);
+        TEST_CASE(getConfigsU7);
 
         TEST_CASE(validateCfg);
 
@@ -2121,6 +2122,14 @@ private:
         ASSERT_EQUALS("\nX=0\n", getConfigsStr(filedata));
     }
 
+    void getConfigsU7() {
+        const char code[] = "#ifndef Y\n"
+                            "#else\n"
+                            "#endif\n";
+        ASSERT_EQUALS("\nY\n", getConfigsStr(code, "-DX"));
+    }
+
+
     void validateCfg() {
         Settings settings;
         Preprocessor preprocessor(settings, this);