Fix seg fault at initalization in debug mode

When using gcc 4.8 on centos7, with debug compile, in a certain app, we
get core dumps involving SIGSEGV in __memcmp_sse4_1 deep inside
operator[](std::string&&).

We don't understand it, and in fact suspect a compiler bug. Can't
reproduce on other platforms or compiler releases. Sweep it under the
rug with this patch, that seems to work around it, though we admit we
don't know why.

src/libOpenImageIO/exif.cpp

@@ -280,8 +280,14 @@ class TagMap {
         for (int i = 0;  tag_table[i].tifftag >= 0;  ++i) {
             const EXIF_tag_info *eti = &tag_table[i];
             m_tagmap[eti->tifftag] = eti;
-            if (eti->name)
-                m_namemap[eti->name] = eti;
+            if (eti->name) {
+                 // m_namemap[eti->name] = eti leads to segfault at
+                 // initialization in debug build on rhel7. This two-step
+                 // process Seems to work. We have no idea why. Maybe gcc
+                 // 4.8 compiler bug?
+                 std::string key = eti->name;
+                 m_namemap[key] = eti;
+            }
         }
     }