memory/patcher: fix coverity warning

Fix CID 1358512:  Error handling issues  (NEGATIVE_RETURNS):

C libraries usually handle read (-1, ...) fine but it is safer to
avoid calling read with a negative handle. Added negative file
descriptor check.

Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>

opal/mca/memory/patcher/memory_patcher_component.c

@@ -277,7 +277,9 @@ static size_t memory_patcher_get_shm_seg_size (const void *shmaddr)
     seg_size = 0;
 
     fd = open ("/proc/self/maps", O_RDONLY);
-    assert (fd >= 0);
+    if (fd < 0) {
+        return 0;
+    }
 
     for (size_t read_offset = 0 ; ; ) {
         ssize_t nread = read(fd, buffer + read_offset, sizeof(buffer) - 1 - read_offset);