Modify/Add test cases to 21-13

Author: jeongsoolee09

c/misra/test/rules/RULE-21-13/test.c

@@ -5,22 +5,33 @@ void sample() {
   unsigned char c1 = 'c';
   int r1 = isalnum(
       c1); // COMPLIANT: ASCII 99 is within unsigned char range of [0, 255]
-  unsigned char x1 = EOF;
-  unsigned char x2 = x1;
-  unsigned char c2 = x2;
-  int r2 = isdigit(c2); // COMPLIANT: EOF (-1)
+  int r2 = isdigit(EOF); // COMPLIANT: EOF (-1)
 
   int x3 = 256;
   int x4 = x3;
   int c3 = x4;
   int r3 =
-      islower(c3); // NON_COMPLIANT: is outside unsigned char range of[0, 255]
+      islower(c3); // NON_COMPLIANT: is outside unsigned char range of [0, 255]
 
   unsigned char x5 = EOF;
   unsigned char x6 = x5;
   int c4 = x6 + 10000;
   int r4 =
-      isdigit(c4); // NON_COMPLIANT: is outside unsigned char range of[0, 255]
+      isdigit(c4); // NON_COMPLIANT: is outside unsigned char range of [0, 255]
+
+  int c5 = getchar();
+  int r5 = isdigit(c5); // COMPLIANT[FALSE_POSITIVE]: library functions like
+                        // getchar needs to be modelled
+
+  unsigned char x7 = 1;
+  char c6;
+  if (x7 == 1) {
+    c6 = EOF;
+  } else {
+    c6 = 'c';
+  }
+  int r6 =
+      isdigit(c6); // COMPLIANT: either control branch make this call compliant.
 }
 
 int main() { return 0; }