IDEA-138441 Methods named like getters are wrongly treated as pure

don't track state for primitive get* methods
for reference-typed methods, track state only inside "if (getX() != null) ..."
flush dfa variables on non-pure getter-like calls

java/java-tests/testData/inspection/dataFlow/fixture/FalseGetters.java

@@ -0,0 +1,21 @@
+class A {
+  private String s;
+  private int next = 0;
+
+  public A(final String s) {
+    this.s = s;
+  }
+
+  private char getChar() {
+    return s.charAt(next++);
+  }
+
+  private void foo() {
+    char c = getChar();
+    if (c == 'a') {
+      if (getChar() == 'b') {
+        System.out.println("ab");
+      }
+    }
+  }
+}