Fix snprintf in minimal-printf library.

mbed_minimal_putchar assumed that buffer being NULL meant that it
should print to a file. This caused a system crash when calling
snprintf with both buffer and stream set to NULL.
It is valid to call snprintf with a NULL buffer; nothing should
be outputted, but the string length should be measured.

TESTS/mbed_platform/minimal-printf/compliance/main.cpp

@@ -635,6 +635,11 @@ static control_t test_snprintf_d(const size_t call_count)
     TEST_ASSERT_EQUAL_INT(result_baseline, result_minimal);
 #endif
 
+    int a = 2;
+    int b = 3;
+    result_minimal = mbed_snprintf(0, 0, "%d + %d = %d\n", a, b, a + b);
+    TEST_ASSERT_EQUAL_INT(10, result_minimal);
+
     return CaseNext;
 }
 

platform/source/minimal-printf/mbed_printf_implementation.c

@@ -122,21 +122,23 @@ static void mbed_minimal_putchar(char *buffer, size_t length, int *result, char
 {
     /* only continue if 'result' doesn't overflow */
     if ((*result >= 0) && (*result <= INT_MAX - 1)) {
-        if (buffer) {
+        if (stream) {
-            /* write data only if there's enough space */
-            if ((size_t)*result < length) {
-                buffer[*result] = data;
-            }
-
-            /* increment 'result' even if data was not written. This ensures that
-               'mbed_minimal_formatted_string' returns the correct value. */
-            *result += 1;
-        } else {
             if (fputc(data, stream) == EOF) {
                 *result = EOF;
             } else {
                 *result += 1;
             }
+        } else {
+            if (buffer) {
+                /* write data only if there's enough space */
+                if ((size_t)*result < length) {
+                    buffer[*result] = data;
+                }
+            }
+
+            /* increment 'result' even if data was not written. This ensures that
+               'mbed_minimal_formatted_string' returns the correct value. */
+            *result += 1;
         }
     }
 }