Add null check for fgets in BufferQueueCore

When compiling BufferQueueCore for host linux, the following
warning/error occurs:

error: ignoring return value of function declared with
'warn_unused_result' attribute [-Werror,-Wunused-result]

Add a simple null check to avoid this error. This is a no-op in terms of
functionality.

Bug: 156675939
Test: m -j libgui
Change-Id: I460731ebaf0272fd5376f244e65b763da44e14f0
diff --git a/libs/gui/BufferQueueCore.cpp b/libs/gui/BufferQueueCore.cpp
index 8d80833..5023b6b 100644
--- a/libs/gui/BufferQueueCore.cpp
+++ b/libs/gui/BufferQueueCore.cpp
@@ -75,10 +75,12 @@
     if (NULL != fp) {
         const size_t size = 64;
         char proc_name[size];
-        fgets(proc_name, size, fp);
+        char* result = fgets(proc_name, size, fp);
         fclose(fp);
-        name = proc_name;
-        return NO_ERROR;
+        if (result != nullptr) {
+            name = proc_name;
+            return NO_ERROR;
+        }
     }
     return INVALID_OPERATION;
 }