glsl: raise GL_INVALID_OPERATION for glUniform(location < -1)

location = -1 is silently ignored, but other negative values should raise
an error.

Another fix for bug 20056.
diff --git a/src/mesa/shader/shader_api.c b/src/mesa/shader/shader_api.c
index 290717f..e738cde 100644
--- a/src/mesa/shader/shader_api.c
+++ b/src/mesa/shader/shader_api.c
@@ -1700,6 +1700,11 @@
    if (location == -1)
       return;   /* The standard specifies this as a no-op */
 
+   if (location < -1) {
+      _mesa_error(ctx, GL_INVALID_OPERATION, "glUniform(location)");
+      return;
+   }
+
    split_location_offset(&location, &offset);
 
    if (location < 0 || location >= (GLint) shProg->Uniforms->NumUniforms) {
@@ -1874,6 +1879,11 @@
    if (location == -1)
       return;   /* The standard specifies this as a no-op */
 
+   if (location < -1) {
+      _mesa_error(ctx, GL_INVALID_OPERATION, "glUniformMatrix(location)");
+      return;
+   }
+
    split_location_offset(&location, &offset);
 
    if (location < 0 || location >= (GLint) shProg->Uniforms->NumUniforms) {